mm: Page fault enhancements

PATCH Superseded Peter Xu 4 participants 20 messages Sep 23, 2019 – Oct 9, 2019

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Peter Xu

author reviewer

2019-09-23 04:25 UTC #0 lore raw

v4 cover mm: Page fault enhancements

This is the 4th version of the PF enhancement series on signal handlings and fault retries. This new version does not change existing patches in v3 but added two more patches to address the current gup issue on not responding to SIGKILL. A 3rd new patch is also added to allow handle_userfaultfd to respect FAULT_FLAG_KILLABLE though should have no functional change when with the two new patches above.

I would really appreciate any review comments for the series, especially for the first two patches which IMHO are even not related to this patchset and they should either cleanup or fix things.

v4:
- use lore.kernel.org for all the links in commit messages [Kirill]
- one more patch ("mm/gup: Fix __get_user_pages() on fault retry of hugetlb") to fix hugetlb path on fault retry
- one more patch ("mm/gup: Allow to react to fatal signals") to:
- use down_read_killable() properly [Linus]
- pass in FAULT_FLAG_KILLABLE for all GUP [Linus]
- one more patch ("mm/userfaultfd: Honor FAULT_FLAG_KILLABLE in fault path") to let handle_userfaultfd() respect FAULT_FLAG_KILLABLE. Should have no functional change after previous two new patches.

v3:
- check fatal signals in __get_user_page_locked() [Linus]
- add r-bs

v2:
- resent previous version, rebase only

=============== v1 cover letter ==================

This series is split out of userfaultfd-wp series to only cover the general page fault changes, since it seems to make sense itself.

Basically it does two things:

(a) Allows the page fault handlers to be more interactive on not only SIGKILL, but also the rest of userspace signals (especially for user-mode faults), and,

(b) Allows the page fault retry (VM_FAULT_RETRY) to happen for more than once.

I'm keeping the CC list as in uffd-wp v5, hopefully I'm not sending too much spams...

And, instead of writting again the cover letter, I'm just copy-pasting my previous link here which has more details on why we do this:

https://patchwork.kernel.org/cover/10691991/

The major change from that latest version should be that we introduced a new page fault flag FAULT_FLAG_INTERRUPTIBLE as suggested by Linus
[1] to represents that we would like the fault handler to respond to non-fatal signals. Also, we're more careful now on when to do the immediate return of the page fault for such signals. For example, now we'll only check against signal_pending() for user-mode page faults and we keep the kernel-mode page fault patch untouched for it. More information can be found in separate patches.

The patchset is only lightly tested on x86.

All comments are greatly welcomed. Thanks,

[1] https://lkml.org/lkml/2019/6/25/1382

Peter Xu (10):
mm/gup: Rename "nonblocking" to "locked" where proper mm/gup: Fix __get_user_pages() on fault retry of hugetlb
mm: Introduce FAULT_FLAG_DEFAULT
mm: Introduce FAULT_FLAG_INTERRUPTIBLE
mm: Return faster for non-fatal signals in user mode faults
userfaultfd: Don't retake mmap_sem to emulate NOPAGE
mm: Allow VM_FAULT_RETRY for multiple times mm/gup: Allow VM_FAULT_RETRY for multiple times mm/gup: Allow to react to fatal signals mm/userfaultfd: Honor FAULT_FLAG_KILLABLE in fault path

 arch/alpha/mm/fault.c           |  7 +--
 arch/arc/mm/fault.c             |  8 ++-
 arch/arm/mm/fault.c             | 14 +++--
 arch/arm64/mm/fault.c           | 16 +++---
 arch/hexagon/mm/vm_fault.c      |  6 +--
 arch/ia64/mm/fault.c            |  6 +--
 arch/m68k/mm/fault.c            | 10 ++--
 arch/microblaze/mm/fault.c      |  6 +--
 arch/mips/mm/fault.c            |  6 +--
 arch/nds32/mm/fault.c           | 12 ++---
 arch/nios2/mm/fault.c           |  8 ++-
 arch/openrisc/mm/fault.c        |  6 +--
 arch/parisc/mm/fault.c          |  9 ++--
 arch/powerpc/mm/fault.c         | 10 ++--
 arch/riscv/mm/fault.c           | 12 ++---
 arch/s390/mm/fault.c            | 11 ++--
 arch/sh/mm/fault.c              |  7 ++-
 arch/sparc/mm/fault_32.c        |  5 +-
 arch/sparc/mm/fault_64.c        |  6 +--
 arch/um/kernel/trap.c           |  7 +--
 arch/unicore32/mm/fault.c       | 11 ++--
 arch/x86/mm/fault.c             |  6 +--
 arch/xtensa/mm/fault.c          |  6 +--
 drivers/gpu/drm/ttm/ttm_bo_vm.c | 12 +++--
 fs/userfaultfd.c                | 62 ++++++++++------------
 include/linux/mm.h              | 81 ++++++++++++++++++++++++----
 include/linux/sched/signal.h    | 12 +++++
 mm/filemap.c                    |  2 +-
 mm/gup.c                        | 93 +++++++++++++++++++++------------
 mm/hugetlb.c                    | 17 +++---
 mm/shmem.c                      |  2 +-
 31 files changed, 283 insertions(+), 193 deletions(-)

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:25 UTC #1 lore raw

PATCH v4 1/10 mm/gup: Rename "nonblocking" to "locked" where proper

There's plenty of places around __get_user_pages() that has a parameter "nonblocking" which does not really mean that "it won't block" (because it can really block) but instead it shows whether the mmap_sem is released by up_read() during the page fault handling mostly when VM_FAULT_RETRY is returned.

We have the correct naming in e.g. get_user_pages_locked() or get_user_pages_remote() as "locked", however there're still many places that are using the "nonblocking" as name.

Renaming the places to "locked" where proper to better suite the functionality of the variable. While at it, fixing up some of the comments accordingly.

Reviewed-by Mike Rapoport <[email protected]>

Reviewed-by Jerome Glisse <[email protected]>

Reviewed-by David Hildenbrand <[email protected]>

Signed-off-by Peter Xu <[email protected]>

---
 mm/gup.c     | 44 +++++++++++++++++++++-----------------------
 mm/hugetlb.c |  8 ++++----
 2 files changed, 25 insertions(+), 27 deletions(-)

mm/gup.c +21 ~~-23~~

index 98f13ab37bac..eddbb95dcb8f 100644
@@ -622,12 +622,12 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address,
 }
 
 /*
- * mmap_sem must be held on entry.  If @nonblocking != NULL and
- * *@flags does not include FOLL_NOWAIT, the mmap_sem may be released.
- * If it is, *@nonblocking will be set to 0 and -EBUSY returned.
+ * mmap_sem must be held on entry.  If @locked != NULL and *@flags
+ * does not include FOLL_NOWAIT, the mmap_sem may be released.  If it
+ * is, *@locked will be set to 0 and -EBUSY returned.
  */
 static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
-		unsigned long address, unsigned int *flags, int *nonblocking)
+		unsigned long address, unsigned int *flags, int *locked)
 {
 	unsigned int fault_flags = 0;
 	vm_fault_t ret;

@@ -639,7 +639,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 		fault_flags |= FAULT_FLAG_WRITE;
 	if (*flags & FOLL_REMOTE)
 		fault_flags |= FAULT_FLAG_REMOTE;
-	if (nonblocking)
+	if (locked)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;

@@ -665,8 +665,8 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 	}
 
 	if (ret & VM_FAULT_RETRY) {
-		if (nonblocking && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
-			*nonblocking = 0;
+		if (locked && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
+			*locked = 0;
 		return -EBUSY;
 	}

@@ -743,7 +743,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
  *		only intends to ensure the pages are faulted in.
  * @vmas:	array of pointers to vmas corresponding to each page.
  *		Or NULL if the caller does not require them.
- * @nonblocking: whether waiting for disk IO or mmap_sem contention
+ * @locked:     whether we're still with the mmap_sem held
  *
  * Returns number of pages pinned. This may be fewer than the number
  * requested. If nr_pages is 0 or negative, returns 0. If no pages

@@ -772,13 +772,11 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
  * appropriate) must be called after the page is finished with, and
  * before put_page is called.
  *
- * If @nonblocking != NULL, __get_user_pages will not wait for disk IO
- * or mmap_sem contention, and if waiting is needed to pin all pages,
- * *@nonblocking will be set to 0.  Further, if @gup_flags does not
- * include FOLL_NOWAIT, the mmap_sem will be released via up_read() in
- * this case.
+ * If @locked != NULL, *@locked will be set to 0 when mmap_sem is
+ * released by an up_read().  That can happen if @gup_flags does not
+ * have FOLL_NOWAIT.
  *
- * A caller using such a combination of @nonblocking and @gup_flags
+ * A caller using such a combination of @locked and @gup_flags
  * must therefore hold the mmap_sem for reading only, and recognize
  * when it's been released.  Otherwise, it must be held for either
  * reading or writing and will not be released.

@@ -790,7 +788,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		unsigned long start, unsigned long nr_pages,
 		unsigned int gup_flags, struct page **pages,
-		struct vm_area_struct **vmas, int *nonblocking)
+		struct vm_area_struct **vmas, int *locked)
 {
 	long ret = 0, i = 0;
 	struct vm_area_struct *vma = NULL;

@@ -834,7 +832,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 			if (is_vm_hugetlb_page(vma)) {
 				i = follow_hugetlb_page(mm, vma, pages, vmas,
 						&start, &nr_pages, i,
-						gup_flags, nonblocking);
+						gup_flags, locked);
 				continue;
 			}
 		}

@@ -852,7 +850,7 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		page = follow_page_mask(vma, start, foll_flags, &ctx);
 		if (!page) {
 			ret = faultin_page(tsk, vma, start, &foll_flags,
-					nonblocking);
+					   locked);
 			switch (ret) {
 			case 0:
 				goto retry;

@@ -1178,7 +1176,7 @@ EXPORT_SYMBOL(get_user_pages_remote);
  * @vma:   target vma
  * @start: start address
  * @end:   end address
- * @nonblocking:
+ * @locked: whether the mmap_sem is still held
  *
  * This takes care of mlocking the pages too if VM_LOCKED is set.
  *

@@ -1186,14 +1184,14 @@ EXPORT_SYMBOL(get_user_pages_remote);
  *
  * vma->vm_mm->mmap_sem must be held.
  *
- * If @nonblocking is NULL, it may be held for read or write and will
+ * If @locked is NULL, it may be held for read or write and will
  * be unperturbed.
  *
- * If @nonblocking is non-NULL, it must held for read only and may be
- * released.  If it's released, *@nonblocking will be set to 0.
+ * If @locked is non-NULL, it must held for read only and may be
+ * released.  If it's released, *@locked will be set to 0.
  */
 long populate_vma_page_range(struct vm_area_struct *vma,
-		unsigned long start, unsigned long end, int *nonblocking)
+		unsigned long start, unsigned long end, int *locked)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long nr_pages = (end - start) / PAGE_SIZE;

@@ -1228,7 +1226,7 @@ long populate_vma_page_range(struct vm_area_struct *vma,
 	 * not result in a stack expansion that recurses back here.
 	 */
 	return __get_user_pages(current, mm, start, nr_pages, gup_flags,
-				NULL, NULL, nonblocking);
+				NULL, NULL, locked);
 }
 
 /*

mm/hugetlb.c +4 -4

index 6d7296dd11b8..31c2a6275023 100644
@@ -4270,7 +4270,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			 struct page **pages, struct vm_area_struct **vmas,
 			 unsigned long *position, unsigned long *nr_pages,
-			 long i, unsigned int flags, int *nonblocking)
+			 long i, unsigned int flags, int *locked)
 {
 	unsigned long pfn_offset;
 	unsigned long vaddr = *position;

@@ -4341,7 +4341,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				spin_unlock(ptl);
 			if (flags & FOLL_WRITE)
 				fault_flags |= FAULT_FLAG_WRITE;
-			if (nonblocking)
+			if (locked)
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY;
 			if (flags & FOLL_NOWAIT)
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY |

@@ -4358,9 +4358,9 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				break;
 			}
 			if (ret & VM_FAULT_RETRY) {
-				if (nonblocking &&
+				if (locked &&
 				    !(fault_flags & FAULT_FLAG_RETRY_NOWAIT))
-					*nonblocking = 0;
+					*locked = 0;
 				*nr_pages = 0;
 				/*
 				 * VM_FAULT_RETRY must not return an

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:25 UTC #2 lore raw

PATCH v4 2/10 mm/gup: Fix __get_user_pages() on fault retry of hugetlb

In reply to Peter Xu

When follow_hugetlb_page() returns with *locked==0, it means we've got a VM_FAULT_RETRY within the fauling process and we've released the mmap_sem. When that happens, we should stop and bail out.

Signed-off-by Peter Xu <[email protected]>

---
 mm/gup.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

mm/gup.c +10 -0

index eddbb95dcb8f..e60d32f1674d 100644
@@ -833,6 +833,16 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 				i = follow_hugetlb_page(mm, vma, pages, vmas,
 						&start, &nr_pages, i,
 						gup_flags, locked);
+				if (locked && *locked == 0) {
+					/*
+					 * We've got a VM_FAULT_RETRY
+					 * and we've lost mmap_sem.
+					 * We must stop here.
+					 */
+					BUG_ON(gup_flags & FOLL_NOWAIT);
+					BUG_ON(ret != 0);
+					goto out;
+				}
 				continue;
 			}
 		}

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:26 UTC #3 lore raw

PATCH v4 3/10 mm: Introduce FAULT_FLAG_DEFAULT

In reply to Peter Xu

Although there're tons of arch-specific page fault handlers, most of them are still sharing the same initial value of the page fault flags. Say, merely all of the page fault handlers would allow the fault to be retried, and they also allow the fault to respond to SIGKILL.

Let's define a default value for the fault flags to replace those initial page fault flags that were copied over. With this, it'll be far easier to introduce new fault flag that can be used by all the architectures instead of touching all the archs.

Reviewed-by David Hildenbrand <[email protected]>

Signed-off-by Peter Xu <[email protected]>

---
 arch/alpha/mm/fault.c      | 2 +-
 arch/arc/mm/fault.c        | 2 +-
 arch/arm/mm/fault.c        | 2 +-
 arch/arm64/mm/fault.c      | 2 +-
 arch/hexagon/mm/vm_fault.c | 2 +-
 arch/ia64/mm/fault.c       | 2 +-
 arch/m68k/mm/fault.c       | 2 +-
 arch/microblaze/mm/fault.c | 2 +-
 arch/mips/mm/fault.c       | 2 +-
 arch/nds32/mm/fault.c      | 2 +-
 arch/nios2/mm/fault.c      | 2 +-
 arch/openrisc/mm/fault.c   | 2 +-
 arch/parisc/mm/fault.c     | 2 +-
 arch/powerpc/mm/fault.c    | 2 +-
 arch/riscv/mm/fault.c      | 2 +-
 arch/s390/mm/fault.c       | 2 +-
 arch/sh/mm/fault.c         | 2 +-
 arch/sparc/mm/fault_32.c   | 2 +-
 arch/sparc/mm/fault_64.c   | 2 +-
 arch/um/kernel/trap.c      | 2 +-
 arch/unicore32/mm/fault.c  | 2 +-
 arch/x86/mm/fault.c        | 2 +-
 arch/xtensa/mm/fault.c     | 2 +-
 include/linux/mm.h         | 7 +++++++
 24 files changed, 30 insertions(+), 23 deletions(-)

arch/alpha/mm/fault.c +1 -1

index 741e61ef9d3f..de4cc6936391 100644
@@ -89,7 +89,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	const struct exception_table_entry *fixup;
 	int si_code = SEGV_MAPERR;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	/* As of EV6, a load into $31/$f31 is a prefetch, and never faults
 	   (or is suppressed by the PALcode).  Support that for older CPUs

arch/arc/mm/fault.c +1 -1

index 3861543b66a0..61919e4e4eec 100644
@@ -94,7 +94,7 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 	         (regs->ecr_cause == ECR_C_PROTV_INST_FETCH))
 		exec = 1;
 
-	flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	flags = FAULT_FLAG_DEFAULT;
 	if (user_mode(regs))
 		flags |= FAULT_FLAG_USER;
 	if (write)

arch/arm/mm/fault.c +1 -1

index 890eeaac3cbb..2ae28ffec622 100644
@@ -241,7 +241,7 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	struct mm_struct *mm;
 	int sig, code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	if (kprobe_page_fault(regs, fsr))
 		return 0;

arch/arm64/mm/fault.c +1 -1

index cfd65b63f36f..613e7434c208 100644
@@ -410,7 +410,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 	struct mm_struct *mm = current->mm;
 	vm_fault_t fault, major = 0;
 	unsigned long vm_flags = VM_READ | VM_WRITE;
-	unsigned int mm_flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int mm_flags = FAULT_FLAG_DEFAULT;
 
 	if (kprobe_page_fault(regs, esr))
 		return 0;

arch/hexagon/mm/vm_fault.c +1 -1

index b3bc71680ae4..223787e01bdd 100644
@@ -41,7 +41,7 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 	int si_code = SEGV_MAPERR;
 	vm_fault_t fault;
 	const struct exception_table_entry *fixup;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	/*
 	 * If we're in an interrupt or have no user context,

arch/ia64/mm/fault.c +1 -1

index c2f299fe9e04..d039b846f671 100644
@@ -65,7 +65,7 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 	struct mm_struct *mm = current->mm;
 	unsigned long mask;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	mask = ((((isr >> IA64_ISR_X_BIT) & 1UL) << VM_EXEC_BIT)
 		| (((isr >> IA64_ISR_W_BIT) & 1UL) << VM_WRITE_BIT));

arch/m68k/mm/fault.c +1 -1

index e9b1d7585b43..8e734309ace9 100644
@@ -71,7 +71,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct * vma;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	pr_debug("do page fault:\nregs->sr=%#x, regs->pc=%#lx, address=%#lx, %ld, %p\n",
 		regs->sr, regs->pc, address, error_code, mm ? mm->pgd : NULL);

arch/microblaze/mm/fault.c +1 -1

index e6a810b0c7ad..45c9f66c1dbc 100644
@@ -91,7 +91,7 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	int code = SEGV_MAPERR;
 	int is_write = error_code & ESR_S;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	regs->ear = address;
 	regs->esr = error_code;

arch/mips/mm/fault.c +1 -1

index f589aa8f47d9..6660b77ff8f3 100644
@@ -44,7 +44,7 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 	const int field = sizeof(unsigned long) * 2;
 	int si_code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	static DEFINE_RATELIMIT_STATE(ratelimit_state, 5 * HZ, 10);

arch/nds32/mm/fault.c +1 -1

index 064ae5d2159d..a40de112a23a 100644
@@ -76,7 +76,7 @@ void do_page_fault(unsigned long entry, unsigned long addr,
 	int si_code;
 	vm_fault_t fault;
 	unsigned int mask = VM_READ | VM_WRITE | VM_EXEC;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	error_code = error_code & (ITYPE_mskINST | ITYPE_mskETYPE);
 	tsk = current;

arch/nios2/mm/fault.c +1 -1

index 6a2e716b959f..a401b45cae47 100644
@@ -47,7 +47,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 	struct mm_struct *mm = tsk->mm;
 	int code = SEGV_MAPERR;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	cause >>= 2;

arch/openrisc/mm/fault.c +1 -1

index 5d4d3a9691d0..fd1592a56238 100644
@@ -50,7 +50,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 	struct vm_area_struct *vma;
 	int si_code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	tsk = current;

arch/parisc/mm/fault.c +1 -1

index adbd5e2144a3..355e3e13fa72 100644
@@ -274,7 +274,7 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 	if (!mm)
 		goto no_context;
 
-	flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	flags = FAULT_FLAG_DEFAULT;
 	if (user_mode(regs))
 		flags |= FAULT_FLAG_USER;

arch/powerpc/mm/fault.c +1 -1

index 8432c281de92..408ee769c470 100644
@@ -435,7 +435,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 {
 	struct vm_area_struct * vma;
 	struct mm_struct *mm = current->mm;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
  	int is_exec = TRAP(regs) == 0x400;
 	int is_user = user_mode(regs);
 	int is_write = page_fault_is_write(error_code);

arch/riscv/mm/fault.c +1 -1

index 96add1427a75..deeb820bd855 100644
@@ -28,7 +28,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	struct vm_area_struct *vma;
 	struct mm_struct *mm;
 	unsigned long addr, cause;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 	int code = SEGV_MAPERR;
 	vm_fault_t fault;

arch/s390/mm/fault.c +1 -1

index 7b0bb475c166..74a77b2bca75 100644
@@ -429,7 +429,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 
 	address = trans_exc_code & __FAIL_ADDR_MASK;
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
-	flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	flags = FAULT_FLAG_DEFAULT;
 	if (user_mode(regs))
 		flags |= FAULT_FLAG_USER;
 	if (access == VM_WRITE || (trans_exc_code & store_indication) == 0x400)

arch/sh/mm/fault.c +1 -1

index 5f51456f4fc7..becf0be267bb 100644
@@ -380,7 +380,7 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 	struct mm_struct *mm;
 	struct vm_area_struct * vma;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	tsk = current;
 	mm = tsk->mm;

arch/sparc/mm/fault_32.c +1 -1

index 8d69de111470..0863f6fdd2c5 100644
@@ -168,7 +168,7 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 	int from_user = !(regs->psr & PSR_PS);
 	int code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	if (text_fault)
 		address = regs->pc;

arch/sparc/mm/fault_64.c +1 -1

index 2371fb6b97e4..a1cba3eef79e 100644
@@ -267,7 +267,7 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 	int si_code, fault_code;
 	vm_fault_t fault;
 	unsigned long address, mm_rss;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	fault_code = get_thread_fault_code();

arch/um/kernel/trap.c +1 -1

index 58fe36856182..bc2756782d64 100644
@@ -32,7 +32,7 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 	pmd_t *pmd;
 	pte_t *pte;
 	int err = -EFAULT;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	*code_out = SEGV_MAPERR;

arch/unicore32/mm/fault.c +1 -1

index 76342de9cf8c..60453c892c51 100644
@@ -202,7 +202,7 @@ static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	struct mm_struct *mm;
 	int sig, code;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	tsk = current;
 	mm = tsk->mm;

arch/x86/mm/fault.c +1 -1

index 9ceacd1156db..994c860ac2d8 100644
@@ -1287,7 +1287,7 @@ void do_user_addr_fault(struct pt_regs *regs,
 	struct task_struct *tsk;
 	struct mm_struct *mm;
 	vm_fault_t fault, major = 0;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	tsk = current;
 	mm = tsk->mm;

arch/xtensa/mm/fault.c +1 -1

index f81b1478da61..d2b082908538 100644
@@ -43,7 +43,7 @@ void do_page_fault(struct pt_regs *regs)
 
 	int is_write, is_exec;
 	vm_fault_t fault;
-	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+	unsigned int flags = FAULT_FLAG_DEFAULT;
 
 	code = SEGV_MAPERR;

include/linux/mm.h +7 -0

index 0334ca97c584..57fb5c535f8e 100644
@@ -393,6 +393,13 @@ extern pgprot_t protection_map[16];
 #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
 #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
 
+/*
+ * The default fault flags that should be used by most of the
+ * arch-specific page fault handlers.
+ */
+#define FAULT_FLAG_DEFAULT  (FAULT_FLAG_ALLOW_RETRY | \
+			     FAULT_FLAG_KILLABLE)
+
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
 	{ FAULT_FLAG_MKWRITE,		"MKWRITE" }, \

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:26 UTC #4 lore raw

PATCH v4 4/10 mm: Introduce FAULT_FLAG_INTERRUPTIBLE

In reply to Peter Xu

handle_userfaultfd() is currently the only one place in the kernel page fault procedures that can respond to non-fatal userspace signals. It was trying to detect such an allowance by checking against USER & KILLABLE flags, which was "un-official".

In this patch, we introduced a new flag (FAULT_FLAG_INTERRUPTIBLE) to show that the fault handler allows the fault procedure to respond even to non-fatal signals. Meanwhile, add this new flag to the default fault flags so that all the page fault handlers can benefit from the new flag. With that, replacing the userfault check to this one.

Since the line is getting even longer, clean up the fault flags a bit too to ease TTY users.

Although we've got a new flag and applied it, we shouldn't have any functional change with this patch so far.

Reviewed-by David Hildenbrand <[email protected]>

Signed-off-by Peter Xu <[email protected]>

---
 fs/userfaultfd.c   |  4 +---
 include/linux/mm.h | 39 ++++++++++++++++++++++++++++-----------
 2 files changed, 29 insertions(+), 14 deletions(-)

fs/userfaultfd.c +1 -3

index fe6d804a38dc..3c55ee64bcb1 100644
@@ -462,9 +462,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	uwq.ctx = ctx;
 	uwq.waken = false;
 
-	return_to_userland =
-		(vmf->flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
-		(FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
+	return_to_userland = vmf->flags & FAULT_FLAG_INTERRUPTIBLE;
 	blocking_state = return_to_userland ? TASK_INTERRUPTIBLE :
 			 TASK_KILLABLE;

include/linux/mm.h +28 ~~-11~~

index 57fb5c535f8e..53ec7abb8472 100644
@@ -383,22 +383,38 @@ extern unsigned int kobjsize(const void *objp);
  */
 extern pgprot_t protection_map[16];
 
-#define FAULT_FLAG_WRITE	0x01	/* Fault was a write access */
-#define FAULT_FLAG_MKWRITE	0x02	/* Fault was mkwrite of existing pte */
-#define FAULT_FLAG_ALLOW_RETRY	0x04	/* Retry fault if blocking */
-#define FAULT_FLAG_RETRY_NOWAIT	0x08	/* Don't drop mmap_sem and wait when retrying */
-#define FAULT_FLAG_KILLABLE	0x10	/* The fault task is in SIGKILL killable region */
-#define FAULT_FLAG_TRIED	0x20	/* Second try */
-#define FAULT_FLAG_USER		0x40	/* The fault originated in userspace */
-#define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
-#define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
+/**
+ * Fault flag definitions.
+ *
+ * @FAULT_FLAG_WRITE: Fault was a write fault.
+ * @FAULT_FLAG_MKWRITE: Fault was mkwrite of existing PTE.
+ * @FAULT_FLAG_ALLOW_RETRY: Allow to retry the fault if blocked.
+ * @FAULT_FLAG_RETRY_NOWAIT: Don't drop mmap_sem and wait when retrying.
+ * @FAULT_FLAG_KILLABLE: The fault task is in SIGKILL killable region.
+ * @FAULT_FLAG_TRIED: The fault has been tried once.
+ * @FAULT_FLAG_USER: The fault originated in userspace.
+ * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
+ * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
+ * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
+ */
+#define FAULT_FLAG_WRITE			0x01
+#define FAULT_FLAG_MKWRITE			0x02
+#define FAULT_FLAG_ALLOW_RETRY			0x04
+#define FAULT_FLAG_RETRY_NOWAIT			0x08
+#define FAULT_FLAG_KILLABLE			0x10
+#define FAULT_FLAG_TRIED			0x20
+#define FAULT_FLAG_USER				0x40
+#define FAULT_FLAG_REMOTE			0x80
+#define FAULT_FLAG_INSTRUCTION  		0x100
+#define FAULT_FLAG_INTERRUPTIBLE		0x200
 
 /*
  * The default fault flags that should be used by most of the
  * arch-specific page fault handlers.
  */
 #define FAULT_FLAG_DEFAULT  (FAULT_FLAG_ALLOW_RETRY | \
-			     FAULT_FLAG_KILLABLE)
+			     FAULT_FLAG_KILLABLE | \
+			     FAULT_FLAG_INTERRUPTIBLE)
 
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \

@@ -409,7 +425,8 @@ extern pgprot_t protection_map[16];
 	{ FAULT_FLAG_TRIED,		"TRIED" }, \
 	{ FAULT_FLAG_USER,		"USER" }, \
 	{ FAULT_FLAG_REMOTE,		"REMOTE" }, \
-	{ FAULT_FLAG_INSTRUCTION,	"INSTRUCTION" }
+	{ FAULT_FLAG_INSTRUCTION,	"INSTRUCTION" }, \
+	{ FAULT_FLAG_INTERRUPTIBLE,	"INTERRUPTIBLE" }
 
 /*
  * vm_fault is filled by the the pagefault handler and passed to the vma's

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:26 UTC #5 lore raw

PATCH v4 5/10 mm: Return faster for non-fatal signals in user mode faults

In reply to Peter Xu

The idea comes from the upstream discussion between Linus and Andrea:

https://lore.kernel.org/lkml/[email protected]/

A summary to the issue: there was a special path in handle_userfault() in the past that we'll return a VM_FAULT_NOPAGE when we detected non-fatal signals when waiting for userfault handling. We did that by reacquiring the mmap_sem before returning. However that brings a risk in that the vmas might have changed when we retake the mmap_sem and even we could be holding an invalid vma structure.

This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.

Signed-off-by Peter Xu <[email protected]>

---
 arch/alpha/mm/fault.c        |  3 ++-
 arch/arc/mm/fault.c          |  5 +++++
 arch/arm/mm/fault.c          |  9 +++++----
 arch/arm64/mm/fault.c        |  9 +++++----
 arch/hexagon/mm/vm_fault.c   |  3 ++-
 arch/ia64/mm/fault.c         |  3 ++-
 arch/m68k/mm/fault.c         |  5 +++--
 arch/microblaze/mm/fault.c   |  3 ++-
 arch/mips/mm/fault.c         |  3 ++-
 arch/nds32/mm/fault.c        |  9 +++++----
 arch/nios2/mm/fault.c        |  3 ++-
 arch/openrisc/mm/fault.c     |  3 ++-
 arch/parisc/mm/fault.c       |  3 ++-
 arch/powerpc/mm/fault.c      |  2 ++
 arch/riscv/mm/fault.c        |  5 +++--
 arch/s390/mm/fault.c         |  4 ++--
 arch/sh/mm/fault.c           |  4 ++++
 arch/sparc/mm/fault_32.c     |  2 +-
 arch/sparc/mm/fault_64.c     |  3 ++-
 arch/um/kernel/trap.c        |  4 +++-
 arch/unicore32/mm/fault.c    |  5 +++--
 arch/x86/mm/fault.c          |  2 ++
 arch/xtensa/mm/fault.c       |  3 ++-
 include/linux/sched/signal.h | 12 ++++++++++++
 24 files changed, 75 insertions(+), 32 deletions(-)

arch/alpha/mm/fault.c +2 -1

index de4cc6936391..ab1d4212d658 100644
@@ -150,7 +150,8 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	   the fault.  */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/arc/mm/fault.c +5 -0

index 61919e4e4eec..27adf4e608e4 100644
@@ -142,6 +142,11 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 				goto no_context;
 			return;
 		}
+
+		/* Allow user to handle non-fatal signals first */
+		if (signal_pending(current) && user_mode(regs))
+			return;
+
 		/*
 		 * retry state machine
 		 */

arch/arm/mm/fault.c +5 -4

index 2ae28ffec622..f00fb4eafe54 100644
@@ -291,14 +291,15 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
 	fault = __do_page_fault(mm, addr, fsr, flags, tsk);
 
-	/* If we need to retry but a fatal signal is pending, handle the
+	/* If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because
 	 * it would already be released in __lock_page_or_retry in
 	 * mm/filemap.c. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
-		if (!user_mode(regs))
+	if (unlikely(fault & VM_FAULT_RETRY && signal_pending(current))) {
+		if (fatal_signal_pending(current) && !user_mode(regs))
 			goto no_context;
-		return 0;
+		if (user_mode(regs))
+			return 0;
 	}
 
 	/*

arch/arm64/mm/fault.c +5 -4

index 613e7434c208..0d3fe0ea6a70 100644
@@ -479,15 +479,16 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 
 	if (fault & VM_FAULT_RETRY) {
 		/*
-		 * If we need to retry but a fatal signal is pending,
+		 * If we need to retry but a signal is pending, try to
 		 * handle the signal first. We do not need to release
 		 * the mmap_sem because it would already be released
 		 * in __lock_page_or_retry in mm/filemap.c.
 		 */
-		if (fatal_signal_pending(current)) {
-			if (!user_mode(regs))
+		if (signal_pending(current)) {
+			if (fatal_signal_pending(current) && !user_mode(regs))
 				goto no_context;
-			return 0;
+			if (user_mode(regs))
+				return 0;
 		}
 
 		/*

arch/hexagon/mm/vm_fault.c +2 -1

index 223787e01bdd..88a2e5635bfb 100644
@@ -91,7 +91,8 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	/* The most common case -- we are done. */

arch/ia64/mm/fault.c +2 -1

index d039b846f671..8d47acf50fda 100644
@@ -141,7 +141,8 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/m68k/mm/fault.c +3 -2

index 8e734309ace9..103f93ba8139 100644
@@ -138,8 +138,9 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	fault = handle_mm_fault(vma, address, flags);
 	pr_debug("handle_mm_fault returns %x\n", fault);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
-		return 0;
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
+		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)

arch/microblaze/mm/fault.c +2 -1

index 45c9f66c1dbc..8b0615eab4b6 100644
@@ -217,7 +217,8 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/mips/mm/fault.c +2 -1

index 6660b77ff8f3..48aac20a1ded 100644
@@ -154,7 +154,8 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);

arch/nds32/mm/fault.c +5 -4

index a40de112a23a..baa44f9d0b4a 100644
@@ -206,14 +206,15 @@ void do_page_fault(unsigned long entry, unsigned long addr,
 	fault = handle_mm_fault(vma, addr, flags);
 
 	/*
-	 * If we need to retry but a fatal signal is pending, handle the
+	 * If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
-		if (!user_mode(regs))
+	if ((fault & VM_FAULT_RETRY) && signal_pending(current)) {
+		if (fatal_signal_pending(current) && !user_mode(regs))
 			goto no_context;
-		return;
+		if (user_mode(regs))
+			return;
 	}
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/nios2/mm/fault.c +2 -1

index a401b45cae47..f9f178484184 100644
@@ -133,7 +133,8 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/openrisc/mm/fault.c +2 -1

index fd1592a56238..8ba3696dd10c 100644
@@ -161,7 +161,8 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/parisc/mm/fault.c +2 -1

index 355e3e13fa72..163dcb080c7b 100644
@@ -304,7 +304,8 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/powerpc/mm/fault.c +2 -0

index 408ee769c470..d321a6c5fe62 100644
@@ -596,6 +596,8 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 			 */
 			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
+			if (is_user && signal_pending(current))
+				return 0;
 			if (!fatal_signal_pending(current))
 				goto retry;
 		}

arch/riscv/mm/fault.c +3 -2

index deeb820bd855..ea8f301de65b 100644
@@ -111,11 +111,12 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	fault = handle_mm_fault(vma, addr, flags);
 
 	/*
-	 * If we need to retry but a fatal signal is pending, handle the
+	 * If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(tsk))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/s390/mm/fault.c +2 -2

index 74a77b2bca75..3ad77501deef 100644
@@ -480,8 +480,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 	 * the fault.
 	 */
 	fault = handle_mm_fault(vma, address, flags);
-	/* No reason to continue if interrupted by SIGKILL. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs))) {
 		fault = VM_FAULT_SIGNAL;
 		if (flags & FAULT_FLAG_RETRY_NOWAIT)
 			goto out_up;

arch/sh/mm/fault.c +4 -0

index becf0be267bb..f620282a37fd 100644
@@ -489,6 +489,10 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 			 * have already released it in __lock_page_or_retry
 			 * in mm/filemap.c.
 			 */
+
+			if (user_mode(regs) && signal_pending(tsk))
+				return;
+
 			goto retry;
 		}
 	}

arch/sparc/mm/fault_32.c +1 -1

index 0863f6fdd2c5..9af0c3ad50d6 100644
@@ -237,7 +237,7 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) && fault_should_check_signal(from_user))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/sparc/mm/fault_64.c +2 -1

index a1cba3eef79e..566f05f9040b 100644
@@ -421,7 +421,8 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(flags & FAULT_FLAG_USER))
 		goto exit_exception;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/um/kernel/trap.c +3 -1

index bc2756782d64..3c72111f27e9 100644
@@ -76,7 +76,9 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 
 		fault = handle_mm_fault(vma, address, flags);
 
-		if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+
+		if ((fault & VM_FAULT_RETRY) &&
+		    fault_should_check_signal(is_user))
 			goto out_nosemaphore;
 
 		if (unlikely(fault & VM_FAULT_ERROR)) {

arch/unicore32/mm/fault.c +3 -2

index 60453c892c51..04c193439c97 100644
@@ -246,11 +246,12 @@ static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
 	fault = __do_pf(mm, addr, fsr, flags, tsk);
 
-	/* If we need to retry but a fatal signal is pending, handle the
+	/* If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because
 	 * it would already be released in __lock_page_or_retry in
 	 * mm/filemap.c. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return 0;
 
 	if (!(fault & VM_FAULT_ERROR) && (flags & FAULT_FLAG_ALLOW_RETRY)) {

arch/x86/mm/fault.c +2 -0

index 994c860ac2d8..f7836472961e 100644
@@ -1451,6 +1451,8 @@ void do_user_addr_fault(struct pt_regs *regs,
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
 			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
+			if ((flags & FAULT_FLAG_USER) && signal_pending(tsk))
+				return;
 			if (!fatal_signal_pending(tsk))
 				goto retry;
 		}

arch/xtensa/mm/fault.c +2 -1

index d2b082908538..094606676c36 100644
@@ -110,7 +110,8 @@ void do_page_fault(struct pt_regs *regs)
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

include/linux/sched/signal.h +12 -0

index efd8ce7675ed..ccce63f2822d 100644
@@ -377,6 +377,18 @@ static inline int signal_pending_state(long state, struct task_struct *p)
 	return (state & TASK_INTERRUPTIBLE) || __fatal_signal_pending(p);
 }
 
+/*
+ * This should only be used in fault handlers to decide whether we
+ * should stop the current fault routine to handle the signals
+ * instead.  It should normally be used when a signal interrupted a
+ * page fault which can lead to a VM_FAULT_RETRY.
+ */
+static inline bool fault_should_check_signal(bool is_user)
+{
+	return (fatal_signal_pending(current) ||
+		(is_user && signal_pending(current)));
+}
+
 /*
  * Reevaluate whether the task has signals pending delivery.
  * Wake the task if so.

Signature · 1 line

2.21.0

Linus Torvalds

2019-09-23 18:04 UTC #6 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

Quoted 4 lines · Peter Xu

This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.

So I really wish saome other vm person would also review these things, but looking over this series once more, this is the patch I probably like the least.

And the reason I like it the least is that I have a hard time explaining to myself what the code does and why, and why it's so full of this pattern:

arch/alpha/mm/fault.c do_page_fault(unsigned long address, unsigned long mmcsr, · line 155

Quoted 4 lines

arch/alpha/mm/fault.c +2 -1

-       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+       if ((fault & VM_FAULT_RETRY) &&
+           fault_should_check_signal(user_mode(regs)))
                return;

which isn't all that pretty.

Why isn't this just

static bool fault_signal_pending(unsigned int fault_flags, struct
pt_regs *regs)

  {
        return (fault_flags & VM_FAULT_RETRY) &&
                (fatal_signal_pending(current) ||
                 (user_mode(regs) && signal_pending(current)));

}

and then most of the users would be something like

if (fault_signal_pending(fault, regs))
return;

and the exceptions could do their own thing.

Now the code is prettier and more understandable, I feel.

And if something doesn't follow this pattern, maybe it either _should_ follow that pattern or it should just not use the helper but explain why it has an unusual pattern.

Linus

Peter Xu

author reviewer

2019-09-24 02:47 UTC #7 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

Quoted 36 lines · Linus Torvalds

Quoted 4 lines · Peter Xu
This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.
So I really wish saome other vm person would also review these things, but looking over this series once more, this is the patch I probably like the least.
And the reason I like it the least is that I have a hard time explaining to myself what the code does and why, and why it's so full of this pattern:
Review
Quoted 4 lines
(quoted diff) +2 -1
-       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+       if ((fault & VM_FAULT_RETRY) &&
+           fault_should_check_signal(user_mode(regs)))
                return;
which isn't all that pretty.
Why isn't this just
static bool fault_signal_pending(unsigned int fault_flags, struct
pt_regs *regs)
  {
        return (fault_flags & VM_FAULT_RETRY) &&
                (fatal_signal_pending(current) ||
                 (user_mode(regs) && signal_pending(current)));
}
and then most of the users would be something like
if (fault_signal_pending(fault, regs))
return;

Quoted 7 lines

and the exceptions could do their own thing.
Now the code is prettier and more understandable, I feel.
And if something doesn't follow this pattern, maybe it either _should_ follow that pattern or it should just not use the helper but explain why it has an unusual pattern.

I see the point on why this patch is disliked - Yeh it should look better to have a single function to cover the most common cases. Besides, I attempted to squash the extra signal_pending() check into some existing code path but maybe it's not really benefiting much while instead it makes the review even harder. So I plan to isolate those paths out too, from something like:

====================================

arch/arm/mm/fault.c +5 -4

@@ -291,14 +291,15 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
        fault = __do_page_fault(mm, addr, fsr, flags, tsk);
 
-       /* If we need to retry but a fatal signal is pending, handle the
+       /* If we need to retry but a signal is pending, try to handle the
         * signal first. We do not need to release the mmap_sem because
         * it would already be released in __lock_page_or_retry in
         * mm/filemap.c. */
-       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
-               if (!user_mode(regs))
+       if (unlikely(fault & VM_FAULT_RETRY && signal_pending(current))) {
+               if (fatal_signal_pending(current) && !user_mode(regs))
                        goto no_context;
-               return 0;
+               if (user_mode(regs))
+                       return 0;
        }

====================================

into:

====================================

arch/arm/mm/fault.c +5 -0

@@ -301,6 +301,11 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		return 0;
 	}
 
+	/* Fast path to handle user mode signals */
+	if ((fault & VM_FAULT_RETRY) && user_mode(regs) &&
+	    signal_pending(current))
+		return 0;
+
 	/*
 	 * Major/minor page fault accounting is only done on the
 	 * initial attempt. If we go through a retry, it is extremely

====================================

I hope it'll be better with that. A complete patch attached too.

Thanks,

Signature · 1 line

Peter Xu

From 2583226afc24bb51b78cf36484f0c5b064b1f75d Mon Sep 17 00:00:00 2001
From: Peter Xu <[email protected]>
Date: Thu, 1 Nov 2018 09:55:29 +0800
Subject: [PATCH] mm: Return faster for non-fatal signals in user mode faults

The idea comes from the upstream discussion between Linus and Andrea:

https://lore.kernel.org/lkml/[email protected]/

This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.

Signed-off-by Peter Xu <[email protected]>

---
 arch/alpha/mm/fault.c        |  2 +-
 arch/arc/mm/fault.c          |  5 +++++
 arch/arm/mm/fault.c          |  5 +++++
 arch/arm64/mm/fault.c        |  4 ++++
 arch/hexagon/mm/vm_fault.c   |  2 +-
 arch/ia64/mm/fault.c         |  2 +-
 arch/m68k/mm/fault.c         |  2 +-
 arch/microblaze/mm/fault.c   |  2 +-
 arch/mips/mm/fault.c         |  2 +-
 arch/nds32/mm/fault.c        |  5 +++++
 arch/nios2/mm/fault.c        |  2 +-
 arch/openrisc/mm/fault.c     |  2 +-
 arch/parisc/mm/fault.c       |  2 +-
 arch/powerpc/mm/fault.c      |  2 ++
 arch/riscv/mm/fault.c        |  4 ++--
 arch/s390/mm/fault.c         |  3 +--
 arch/sh/mm/fault.c           |  4 ++++
 arch/sparc/mm/fault_32.c     |  2 +-
 arch/sparc/mm/fault_64.c     |  2 +-
 arch/um/kernel/trap.c        |  4 +++-
 arch/unicore32/mm/fault.c    |  4 ++--
 arch/x86/mm/fault.c          |  2 ++
 arch/xtensa/mm/fault.c       |  2 +-
 include/linux/sched/signal.h | 14 ++++++++++++++
 24 files changed, 61 insertions(+), 19 deletions(-)

arch/alpha/mm/fault.c +1 -1

index de4cc6936391..fcfa229cc1e7 100644
@@ -150,7 +150,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	   the fault.  */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/arc/mm/fault.c +5 -0

index 61919e4e4eec..27adf4e608e4 100644
@@ -142,6 +142,11 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 				goto no_context;
 			return;
 		}
+
+		/* Allow user to handle non-fatal signals first */
+		if (signal_pending(current) && user_mode(regs))
+			return;
+
 		/*
 		 * retry state machine
 		 */

arch/arm/mm/fault.c +5 -0

index 2ae28ffec622..44fa64dbb8e0 100644
@@ -301,6 +301,11 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		return 0;
 	}
 
+	/* Fast path to handle user mode signals */
+	if ((fault & VM_FAULT_RETRY) && user_mode(regs) &&
+	    signal_pending(current))
+		return 0;
+
 	/*
 	 * Major/minor page fault accounting is only done on the
 	 * initial attempt. If we go through a retry, it is extremely

arch/arm64/mm/fault.c +4 -0

index 613e7434c208..8ff04af1d982 100644
@@ -490,6 +490,10 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 			return 0;
 		}
 
+		/* Fast path for user mode signals */
+		if (user_mode(regs) && signal_pending(current))
+			return 0;
+
 		/*
 		 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk of
 		 * starvation.

arch/hexagon/mm/vm_fault.c +1 -1

index 223787e01bdd..d9e15d941bdb 100644
@@ -91,7 +91,7 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	/* The most common case -- we are done. */

arch/ia64/mm/fault.c +1 -1

index d039b846f671..b5aa4e80c762 100644
@@ -141,7 +141,7 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/m68k/mm/fault.c +1 -1

index 8e734309ace9..182799fd9987 100644
@@ -138,7 +138,7 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	fault = handle_mm_fault(vma, address, flags);
 	pr_debug("handle_mm_fault returns %x\n", fault);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return 0;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/microblaze/mm/fault.c +1 -1

index 45c9f66c1dbc..32da02778a63 100644
@@ -217,7 +217,7 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/mips/mm/fault.c +1 -1

index 6660b77ff8f3..f811ef5efb80 100644
@@ -154,7 +154,7 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(regs))
 		return;
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);

arch/nds32/mm/fault.c +5 -0

index a40de112a23a..c4c53de68ec3 100644
@@ -216,6 +216,11 @@ void do_page_fault(unsigned long entry, unsigned long addr,
 		return;
 	}
 
+	/* Fast path for user mode signals */
+	if ((fault & VM_FAULT_RETRY) && signal_pending(current) &&
+	    user_mode(regs))
+		return;
+
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
 			goto out_of_memory;

arch/nios2/mm/fault.c +1 -1

index a401b45cae47..c38bea4220fb 100644
@@ -133,7 +133,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/openrisc/mm/fault.c +1 -1

index fd1592a56238..30d5c51e9d40 100644
@@ -161,7 +161,7 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/parisc/mm/fault.c +1 -1

index 355e3e13fa72..8e88e5c5f26a 100644
@@ -304,7 +304,7 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/powerpc/mm/fault.c +2 -0

index 408ee769c470..d321a6c5fe62 100644
@@ -596,6 +596,8 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 			 */
 			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
+			if (is_user && signal_pending(current))
+				return 0;
 			if (!fatal_signal_pending(current))
 				goto retry;
 		}

arch/riscv/mm/fault.c +2 -2

index deeb820bd855..ba652a2a6ad9 100644
@@ -111,11 +111,11 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	fault = handle_mm_fault(vma, addr, flags);
 
 	/*
-	 * If we need to retry but a fatal signal is pending, handle the
+	 * If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(tsk))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/s390/mm/fault.c +1 -2

index 74a77b2bca75..551ac311bd35 100644
@@ -480,8 +480,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 	 * the fault.
 	 */
 	fault = handle_mm_fault(vma, address, flags);
-	/* No reason to continue if interrupted by SIGKILL. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
+	if (fault_signal_pending(fault, regs)) {
 		fault = VM_FAULT_SIGNAL;
 		if (flags & FAULT_FLAG_RETRY_NOWAIT)
 			goto out_up;

arch/sh/mm/fault.c +4 -0

index becf0be267bb..f620282a37fd 100644
@@ -489,6 +489,10 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 			 * have already released it in __lock_page_or_retry
 			 * in mm/filemap.c.
 			 */
+
+			if (user_mode(regs) && signal_pending(tsk))
+				return;
+
 			goto retry;
 		}
 	}

arch/sparc/mm/fault_32.c +1 -1

index 0863f6fdd2c5..447f61c4e996 100644
@@ -237,7 +237,7 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/sparc/mm/fault_64.c +1 -1

index a1cba3eef79e..6807fba66331 100644
@@ -421,7 +421,7 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		goto exit_exception;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/um/kernel/trap.c +3 -1

index bc2756782d64..32a6830c1d45 100644
@@ -76,7 +76,9 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 
 		fault = handle_mm_fault(vma, address, flags);
 
-		if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+                if ((fault & VM_FAULT_RETRY) &&
+		    (fatal_signal_pending(current) ||
+		     (is_user && signal_pending(current))))
 			goto out_nosemaphore;
 
 		if (unlikely(fault & VM_FAULT_ERROR)) {

arch/unicore32/mm/fault.c +2 -2

index 60453c892c51..f24a1967c323 100644
@@ -246,11 +246,11 @@ static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
 	fault = __do_pf(mm, addr, fsr, flags, tsk);
 
-	/* If we need to retry but a fatal signal is pending, handle the
+	/* If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because
 	 * it would already be released in __lock_page_or_retry in
 	 * mm/filemap.c. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return 0;
 
 	if (!(fault & VM_FAULT_ERROR) && (flags & FAULT_FLAG_ALLOW_RETRY)) {

arch/x86/mm/fault.c +2 -0

index 994c860ac2d8..f7836472961e 100644
@@ -1451,6 +1451,8 @@ void do_user_addr_fault(struct pt_regs *regs,
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
 			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
+			if ((flags & FAULT_FLAG_USER) && signal_pending(tsk))
+				return;
 			if (!fatal_signal_pending(tsk))
 				goto retry;
 		}

arch/xtensa/mm/fault.c +1 -1

index d2b082908538..6467e3fb9762 100644
@@ -110,7 +110,7 @@ void do_page_fault(struct pt_regs *regs)
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

include/linux/sched/signal.h +14 -0

index efd8ce7675ed..3f517a30ec4d 100644
@@ -377,6 +377,20 @@ static inline int signal_pending_state(long state, struct task_struct *p)
 	return (state & TASK_INTERRUPTIBLE) || __fatal_signal_pending(p);
 }
 
+/*
+ * This should only be used in fault handlers to decide whether we
+ * should stop the current fault routine to handle the signals
+ * instead.  It should normally be used when a signal interrupted a
+ * page fault which can lead to a VM_FAULT_RETRY.
+ */
+static inline bool fault_signal_pending(unsigned int fault_flags,
+					struct pt_regs *regs)
+{
+	return (fault_flags & VM_FAULT_RETRY) &&
+	    (fatal_signal_pending(current) ||
+	     (user_mode(regs) && signal_pending(current)));
+}
+
 /*
  * Reevaluate whether the task has signals pending delivery.
  * Wake the task if so.

Signature · 1 line

2.21.0

Matthew Wilcox

maintainer

2019-09-24 02:54 UTC #8 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

Quoted 58 lines · Peter Xu

Quoted 36 lines · Linus Torvalds
Quoted 4 lines · Peter Xu
This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.
So I really wish saome other vm person would also review these things, but looking over this series once more, this is the patch I probably like the least.
And the reason I like it the least is that I have a hard time explaining to myself what the code does and why, and why it's so full of this pattern:
arch/um/kernel/trap.c int handle_page_fault(unsigned long address, unsigned long ip,
Quoted 4 lines
arch/um/kernel/trap.c +2 -1
-       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+       if ((fault & VM_FAULT_RETRY) &&
+           fault_should_check_signal(user_mode(regs)))
                return;
which isn't all that pretty.
Why isn't this just
static bool fault_signal_pending(unsigned int fault_flags, struct
pt_regs *regs)
  {
        return (fault_flags & VM_FAULT_RETRY) &&
                (fatal_signal_pending(current) ||
                 (user_mode(regs) && signal_pending(current)));
}
and then most of the users would be something like
if (fault_signal_pending(fault, regs))
return;
Quoted 7 lines
and the exceptions could do their own thing.
Now the code is prettier and more understandable, I feel.
And if something doesn't follow this pattern, maybe it either _should_ follow that pattern or it should just not use the helper but explain why it has an unusual pattern.
+++ b/arch/alpha/mm/fault.c
arch/alpha/mm/fault.c +2 -1
@@ -150,7 +150,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	   the fault.  */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 
+++ b/arch/arm/mm/fault.c

arch/alpha/mm/fault.c do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs) · line 307

Quoted 7 lines

arch/alpha/mm/fault.c +4 -0

@@ -301,6 +301,11 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		return 0;
 	}
 
+	/* Fast path to handle user mode signals */
+	if ((fault & VM_FAULT_RETRY) && user_mode(regs) &&
+	    signal_pending(current))
+		return 0;

But _why_ are they different? This is a good opportunity to make more code the same between architectures.

Peter Xu

author reviewer

2019-09-24 03:19 UTC #9 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

Quoted 62 lines · Matthew Wilcox

Quoted 61 lines · Peter Xu
Quoted 36 lines · Linus Torvalds
Quoted 4 lines · Peter Xu
This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.
So I really wish saome other vm person would also review these things, but looking over this series once more, this is the patch I probably like the least.
And the reason I like it the least is that I have a hard time explaining to myself what the code does and why, and why it's so full of this pattern:
arch/um/kernel/trap.c int handle_page_fault(unsigned long address, unsigned long ip,
Quoted 4 lines
arch/um/kernel/trap.c +2 -1
-       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+       if ((fault & VM_FAULT_RETRY) &&
+           fault_should_check_signal(user_mode(regs)))
                return;
which isn't all that pretty.
Why isn't this just
static bool fault_signal_pending(unsigned int fault_flags, struct
pt_regs *regs)
  {
        return (fault_flags & VM_FAULT_RETRY) &&
                (fatal_signal_pending(current) ||
                 (user_mode(regs) && signal_pending(current)));
}
and then most of the users would be something like
if (fault_signal_pending(fault, regs))
return;
Quoted 7 lines
and the exceptions could do their own thing.
Now the code is prettier and more understandable, I feel.
And if something doesn't follow this pattern, maybe it either _should_ follow that pattern or it should just not use the helper but explain why it has an unusual pattern.
+++ b/arch/alpha/mm/fault.c
arch/alpha/mm/fault.c +2 -1
@@ -150,7 +150,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	   the fault.  */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 
+++ b/arch/arm/mm/fault.c
@@ -301,6 +301,11 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		return 0;
 	}
 

Quoted 7 lines

arch/arm/mm/fault.c do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs) · line 307
Quoted 4 lines
arch/arm/mm/fault.c +4 -0
+	/* Fast path to handle user mode signals */
+	if ((fault & VM_FAULT_RETRY) && user_mode(regs) &&
+	    signal_pending(current))
+		return 0;
But _why_ are they different? This is a good opportunity to make more code the same between architectures.

(Thanks for joining the discussion)

I'd like to do these - my only worry is that I can't really test them well simply because I don't have all the hardwares. For now the changes are mostly straightforward so I'm relatively confident (not to mention the code needs proper reviews too, and of course I would appreciate much if anyone wants to smoke test it). If I change it in a drastic way, I won't be that confident without some tests at least on multiple archs (not to mention that even smoke testing across major archs will be a huge amount of work...). So IMHO those might be more suitable as follow-up for per-arch developers if we can at least reach a consensus on the whole idea of this patchset.

Thanks,

Signature · 1 line

Peter Xu

Matthew Wilcox

maintainer

2019-09-24 15:45 UTC #10 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

Quoted 73 lines · Peter Xu

Quoted 62 lines · Matthew Wilcox
Quoted 61 lines · Peter Xu
Quoted 36 lines · Linus Torvalds
Quoted 4 lines · Peter Xu
This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.
So I really wish saome other vm person would also review these things, but looking over this series once more, this is the patch I probably like the least.
And the reason I like it the least is that I have a hard time explaining to myself what the code does and why, and why it's so full of this pattern:
arch/um/kernel/trap.c int handle_page_fault(unsigned long address, unsigned long ip,
Quoted 4 lines
arch/um/kernel/trap.c +2 -1
-       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+       if ((fault & VM_FAULT_RETRY) &&
+           fault_should_check_signal(user_mode(regs)))
                return;
which isn't all that pretty.
Why isn't this just
static bool fault_signal_pending(unsigned int fault_flags, struct
pt_regs *regs)
  {
        return (fault_flags & VM_FAULT_RETRY) &&
                (fatal_signal_pending(current) ||
                 (user_mode(regs) && signal_pending(current)));
}
and then most of the users would be something like
if (fault_signal_pending(fault, regs))
return;
Quoted 7 lines
and the exceptions could do their own thing.
Now the code is prettier and more understandable, I feel.
And if something doesn't follow this pattern, maybe it either _should_ follow that pattern or it should just not use the helper but explain why it has an unusual pattern.
+++ b/arch/alpha/mm/fault.c
arch/alpha/mm/fault.c +2 -1
@@ -150,7 +150,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	   the fault.  */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 
+++ b/arch/arm/mm/fault.c
@@ -301,6 +301,11 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		return 0;
 	}
 
Quoted 7 lines
arch/arm/mm/fault.c do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs) · line 307
Quoted 4 lines
arch/arm/mm/fault.c +4 -0
+	/* Fast path to handle user mode signals */
+	if ((fault & VM_FAULT_RETRY) && user_mode(regs) &&
+	    signal_pending(current))
+		return 0;
But _why_ are they different? This is a good opportunity to make more code the same between architectures.
(Thanks for joining the discussion)

Quoted 10 lines

I'd like to do these - my only worry is that I can't really test them well simply because I don't have all the hardwares. For now the changes are mostly straightforward so I'm relatively confident (not to mention the code needs proper reviews too, and of course I would appreciate much if anyone wants to smoke test it). If I change it in a drastic way, I won't be that confident without some tests at least on multiple archs (not to mention that even smoke testing across major archs will be a huge amount of work...). So IMHO those might be more suitable as follow-up for per-arch developers if we can at least reach a consensus on the whole idea of this patchset.

I think the way to do this is to introduce fault_signal_pending(), converting the architectures to it that match that pattern. Then one patch per architecture to convert the ones which use a different pattern to the same pattern.

Oh, and while you're looking at the callers of handle_mm_fault(), a lot of them don't check conditions in the right order. x86, at least, handles FAULT_RETRY before handling FAULT_ERROR, which is clearly wrong.

Kirill and I recently discussed it here:
https://lore.kernel.org/linux-mm/20190911152338.gqqgxrmqycodfocb@box/T/

Peter Xu

author reviewer

2019-09-25 03:46 UTC #11 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

Quoted 85 lines · Matthew Wilcox

Quoted 73 lines · Peter Xu
Quoted 62 lines · Matthew Wilcox
Quoted 61 lines · Peter Xu
Quoted 36 lines · Linus Torvalds
Quoted 4 lines · Peter Xu
This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.
So I really wish saome other vm person would also review these things, but looking over this series once more, this is the patch I probably like the least.
And the reason I like it the least is that I have a hard time explaining to myself what the code does and why, and why it's so full of this pattern:
arch/um/kernel/trap.c int handle_page_fault(unsigned long address, unsigned long ip,
Quoted 4 lines
arch/um/kernel/trap.c +2 -1
-       if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+       if ((fault & VM_FAULT_RETRY) &&
+           fault_should_check_signal(user_mode(regs)))
                return;
which isn't all that pretty.
Why isn't this just
static bool fault_signal_pending(unsigned int fault_flags, struct
pt_regs *regs)
  {
        return (fault_flags & VM_FAULT_RETRY) &&
                (fatal_signal_pending(current) ||
                 (user_mode(regs) && signal_pending(current)));
}
and then most of the users would be something like
if (fault_signal_pending(fault, regs))
return;
Quoted 7 lines
and the exceptions could do their own thing.
Now the code is prettier and more understandable, I feel.
And if something doesn't follow this pattern, maybe it either _should_ follow that pattern or it should just not use the helper but explain why it has an unusual pattern.
+++ b/arch/alpha/mm/fault.c
arch/alpha/mm/fault.c +2 -1
@@ -150,7 +150,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	   the fault.  */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if (fault_signal_pending(fault, regs))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 
+++ b/arch/arm/mm/fault.c
@@ -301,6 +301,11 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		return 0;
 	}
 
Quoted 7 lines
arch/arm/mm/fault.c do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs) · line 307
Quoted 4 lines
arch/arm/mm/fault.c +4 -0
+	/* Fast path to handle user mode signals */
+	if ((fault & VM_FAULT_RETRY) && user_mode(regs) &&
+	    signal_pending(current))
+		return 0;
But _why_ are they different? This is a good opportunity to make more code the same between architectures.
(Thanks for joining the discussion)
Quoted 10 lines
I'd like to do these - my only worry is that I can't really test them well simply because I don't have all the hardwares. For now the changes are mostly straightforward so I'm relatively confident (not to mention the code needs proper reviews too, and of course I would appreciate much if anyone wants to smoke test it). If I change it in a drastic way, I won't be that confident without some tests at least on multiple archs (not to mention that even smoke testing across major archs will be a huge amount of work...). So IMHO those might be more suitable as follow-up for per-arch developers if we can at least reach a consensus on the whole idea of this patchset.

Quoted 4 lines

I think the way to do this is to introduce fault_signal_pending(), converting the architectures to it that match that pattern. Then one patch per architecture to convert the ones which use a different pattern to the same pattern.

Fair enough. I can start with a fault_signal_pending() only keeps the sigkill handling just like before, then convert all the archs, with the last patch to only touch fault_signal_pending() for non-fatal signals.

Quoted 7 lines

Oh, and while you're looking at the callers of handle_mm_fault(), a lot of them don't check conditions in the right order. x86, at least, handles FAULT_RETRY before handling FAULT_ERROR, which is clearly wrong.
Kirill and I recently discussed it here:
https://lore.kernel.org/linux-mm/20190911152338.gqqgxrmqycodfocb@box/T/

Hmm sure. These sound very reasonable.

I must admit that I am not brave enough to continue grow my patchset on my own. The condition I'm facing right now is that I can't really find enough reviewers for this series (Linus helped me quite a lot, I really, really, appreciated that), while it's still growing. I hope the started discussion means that you'll be at least another potential reviewer (oh, should I count Kirill in as well? :) at least to the coming patches for the things mentioned above.

Thanks,

Signature · 1 line

Peter Xu

Peter Xu

author reviewer

2019-09-26 08:59 UTC #12 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

In reply to Matthew Wilcox

On Tue, Sep 24, 2019 at 08:45:18AM -0700, Matthew Wilcox wrote:

[...]

Quoted 6 lines

Oh, and while you're looking at the callers of handle_mm_fault(), a lot of them don't check conditions in the right order. x86, at least, handles FAULT_RETRY before handling FAULT_ERROR, which is clearly wrong.
Kirill and I recently discussed it here:
https://lore.kernel.org/linux-mm/20190911152338.gqqgxrmqycodfocb@box/T/

Is there any existing path in master that we can get VM_FAULT_RETRY returned with any existing VM_FAULT_ERROR bit? It seems to me that above link is the first one that is going to introduce such case?

If so, I'm uncertain now on whether I should have one patch to handle the ERROR case first as you suggested with this series, because otherwise that patch won't explain itself without a real benefit...

Thanks,

Signature · 1 line

Peter Xu

Palmer Dabbelt

Acked-by on v4 5/10

2019-10-08 22:43 UTC #13 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

In reply to Peter Xu

Quoted 284 lines · On Sun, 22 Sep 2019 21:25:18 PDT (-0700), [email protected] wrote:

The idea comes from the upstream discussion between Linus and Andrea:

https://lore.kernel.org/lkml/[email protected]/

This patch is a preparation of removing that special path by allowing the page fault to return even faster if we were interrupted by a non-fatal signal during a user-mode page fault handling routine.

Signed-off-by Peter Xu <[email protected]>

---
 arch/alpha/mm/fault.c        |  3 ++-
 arch/arc/mm/fault.c          |  5 +++++
 arch/arm/mm/fault.c          |  9 +++++----
 arch/arm64/mm/fault.c        |  9 +++++----
 arch/hexagon/mm/vm_fault.c   |  3 ++-
 arch/ia64/mm/fault.c         |  3 ++-
 arch/m68k/mm/fault.c         |  5 +++--
 arch/microblaze/mm/fault.c   |  3 ++-
 arch/mips/mm/fault.c         |  3 ++-
 arch/nds32/mm/fault.c        |  9 +++++----
 arch/nios2/mm/fault.c        |  3 ++-
 arch/openrisc/mm/fault.c     |  3 ++-
 arch/parisc/mm/fault.c       |  3 ++-
 arch/powerpc/mm/fault.c      |  2 ++
 arch/riscv/mm/fault.c        |  5 +++--
 arch/s390/mm/fault.c         |  4 ++--
 arch/sh/mm/fault.c           |  4 ++++
 arch/sparc/mm/fault_32.c     |  2 +-
 arch/sparc/mm/fault_64.c     |  3 ++-
 arch/um/kernel/trap.c        |  4 +++-
 arch/unicore32/mm/fault.c    |  5 +++--
 arch/x86/mm/fault.c          |  2 ++
 arch/xtensa/mm/fault.c       |  3 ++-
 include/linux/sched/signal.h | 12 ++++++++++++
 24 files changed, 75 insertions(+), 32 deletions(-)

arch/alpha/mm/fault.c +2 -1

index de4cc6936391..ab1d4212d658 100644
@@ -150,7 +150,8 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	   the fault.  */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/arc/mm/fault.c +5 -0

index 61919e4e4eec..27adf4e608e4 100644
@@ -142,6 +142,11 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 				goto no_context;
 			return;
 		}
+
+		/* Allow user to handle non-fatal signals first */
+		if (signal_pending(current) && user_mode(regs))
+			return;
+
 		/*
 		 * retry state machine
 		 */

arch/arm/mm/fault.c +5 -4

index 2ae28ffec622..f00fb4eafe54 100644
@@ -291,14 +291,15 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
 	fault = __do_page_fault(mm, addr, fsr, flags, tsk);
 
-	/* If we need to retry but a fatal signal is pending, handle the
+	/* If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because
 	 * it would already be released in __lock_page_or_retry in
 	 * mm/filemap.c. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
-		if (!user_mode(regs))
+	if (unlikely(fault & VM_FAULT_RETRY && signal_pending(current))) {
+		if (fatal_signal_pending(current) && !user_mode(regs))
 			goto no_context;
-		return 0;
+		if (user_mode(regs))
+			return 0;
 	}
 
 	/*

arch/arm64/mm/fault.c +5 -4

index 613e7434c208..0d3fe0ea6a70 100644
@@ -479,15 +479,16 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 
 	if (fault & VM_FAULT_RETRY) {
 		/*
-		 * If we need to retry but a fatal signal is pending,
+		 * If we need to retry but a signal is pending, try to
 		 * handle the signal first. We do not need to release
 		 * the mmap_sem because it would already be released
 		 * in __lock_page_or_retry in mm/filemap.c.
 		 */
-		if (fatal_signal_pending(current)) {
-			if (!user_mode(regs))
+		if (signal_pending(current)) {
+			if (fatal_signal_pending(current) && !user_mode(regs))
 				goto no_context;
-			return 0;
+			if (user_mode(regs))
+				return 0;
 		}
 
 		/*

arch/hexagon/mm/vm_fault.c +2 -1

index 223787e01bdd..88a2e5635bfb 100644
@@ -91,7 +91,8 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	/* The most common case -- we are done. */

arch/ia64/mm/fault.c +2 -1

index d039b846f671..8d47acf50fda 100644
@@ -141,7 +141,8 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/m68k/mm/fault.c +3 -2

index 8e734309ace9..103f93ba8139 100644
@@ -138,8 +138,9 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 	fault = handle_mm_fault(vma, address, flags);
 	pr_debug("handle_mm_fault returns %x\n", fault);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
-		return 0;
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
+		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)

arch/microblaze/mm/fault.c +2 -1

index 45c9f66c1dbc..8b0615eab4b6 100644
@@ -217,7 +217,8 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/mips/mm/fault.c +2 -1

index 6660b77ff8f3..48aac20a1ded 100644
@@ -154,7 +154,8 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);

arch/nds32/mm/fault.c +5 -4

index a40de112a23a..baa44f9d0b4a 100644
@@ -206,14 +206,15 @@ void do_page_fault(unsigned long entry, unsigned long addr,
 	fault = handle_mm_fault(vma, addr, flags);
 
 	/*
-	 * If we need to retry but a fatal signal is pending, handle the
+	 * If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
-		if (!user_mode(regs))
+	if ((fault & VM_FAULT_RETRY) && signal_pending(current)) {
+		if (fatal_signal_pending(current) && !user_mode(regs))
 			goto no_context;
-		return;
+		if (user_mode(regs))
+			return;
 	}
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/nios2/mm/fault.c +2 -1

index a401b45cae47..f9f178484184 100644
@@ -133,7 +133,8 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/openrisc/mm/fault.c +2 -1

index fd1592a56238..8ba3696dd10c 100644
@@ -161,7 +161,8 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/parisc/mm/fault.c +2 -1

index 355e3e13fa72..163dcb080c7b 100644
@@ -304,7 +304,8 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/powerpc/mm/fault.c +2 -0

index 408ee769c470..d321a6c5fe62 100644
@@ -596,6 +596,8 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 			 */
 			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
+			if (is_user && signal_pending(current))
+				return 0;
 			if (!fatal_signal_pending(current))
 				goto retry;
 		}

arch/riscv/mm/fault.c +0 -1

index deeb820bd855..ea8f301de65b 100644
@@ -111,11 +111,12 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	fault = handle_mm_fault(vma, addr, flags);
 
 	/*
-	 * If we need to retry but a fatal signal is pending, handle the

arch/riscv/mm/fault.c asmlinkage void do_page_fault(struct pt_regs *regs) · line 122

Quoted 10 lines

arch/riscv/mm/fault.c +3 -1

+	 * If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(tsk))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

Acked-by Palmer Dabbelt <[email protected]> # RISC-V parts

I'm assuming this is going in through some other tree.

Quoted 141 lines

arch/s390/mm/fault.c +2 -2

index 74a77b2bca75..3ad77501deef 100644
@@ -480,8 +480,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 	 * the fault.
 	 */
 	fault = handle_mm_fault(vma, address, flags);
-	/* No reason to continue if interrupted by SIGKILL. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) {
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs))) {
 		fault = VM_FAULT_SIGNAL;
 		if (flags & FAULT_FLAG_RETRY_NOWAIT)
 			goto out_up;

arch/sh/mm/fault.c +4 -0

index becf0be267bb..f620282a37fd 100644
@@ -489,6 +489,10 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 			 * have already released it in __lock_page_or_retry
 			 * in mm/filemap.c.
 			 */
+
+			if (user_mode(regs) && signal_pending(tsk))
+				return;
+
 			goto retry;
 		}
 	}

arch/sparc/mm/fault_32.c +1 -1

index 0863f6fdd2c5..9af0c3ad50d6 100644
@@ -237,7 +237,7 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) && fault_should_check_signal(from_user))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/sparc/mm/fault_64.c +2 -1

index a1cba3eef79e..566f05f9040b 100644
@@ -421,7 +421,8 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(flags & FAULT_FLAG_USER))
 		goto exit_exception;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

arch/um/kernel/trap.c +3 -1

index bc2756782d64..3c72111f27e9 100644
@@ -76,7 +76,9 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 
 		fault = handle_mm_fault(vma, address, flags);
 
-		if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+
+		if ((fault & VM_FAULT_RETRY) &&
+		    fault_should_check_signal(is_user))
 			goto out_nosemaphore;
 
 		if (unlikely(fault & VM_FAULT_ERROR)) {

arch/unicore32/mm/fault.c +3 -2

index 60453c892c51..04c193439c97 100644
@@ -246,11 +246,12 @@ static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 
 	fault = __do_pf(mm, addr, fsr, flags, tsk);
 
-	/* If we need to retry but a fatal signal is pending, handle the
+	/* If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because
 	 * it would already be released in __lock_page_or_retry in
 	 * mm/filemap.c. */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return 0;
 
 	if (!(fault & VM_FAULT_ERROR) && (flags & FAULT_FLAG_ALLOW_RETRY)) {

arch/x86/mm/fault.c +2 -0

index 994c860ac2d8..f7836472961e 100644
@@ -1451,6 +1451,8 @@ void do_user_addr_fault(struct pt_regs *regs,
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
 			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
+			if ((flags & FAULT_FLAG_USER) && signal_pending(tsk))
+				return;
 			if (!fatal_signal_pending(tsk))
 				goto retry;
 		}

arch/xtensa/mm/fault.c +2 -1

index d2b082908538..094606676c36 100644
@@ -110,7 +110,8 @@ void do_page_fault(struct pt_regs *regs)
 	 */
 	fault = handle_mm_fault(vma, address, flags);
 
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {

include/linux/sched/signal.h +12 -0

index efd8ce7675ed..ccce63f2822d 100644
@@ -377,6 +377,18 @@ static inline int signal_pending_state(long state, struct task_struct *p)
 	return (state & TASK_INTERRUPTIBLE) || __fatal_signal_pending(p);
 }
 
+/*
+ * This should only be used in fault handlers to decide whether we
+ * should stop the current fault routine to handle the signals
+ * instead.  It should normally be used when a signal interrupted a
+ * page fault which can lead to a VM_FAULT_RETRY.
+ */
+static inline bool fault_should_check_signal(bool is_user)
+{
+	return (fatal_signal_pending(current) ||
+		(is_user && signal_pending(current)));
+}
+
 /*
  * Reevaluate whether the task has signals pending delivery.
  * Wake the task if so.

Peter Xu

author reviewer

2019-10-09 07:41 UTC #14 lore raw

Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults

Quoted 18 lines · Palmer Dabbelt

Quoted 18 lines

arch/riscv/mm/fault.c +3 -2

index deeb820bd855..ea8f301de65b 100644
@@ -111,11 +111,12 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 	fault = handle_mm_fault(vma, addr, flags);
 
 	/*
-	 * If we need to retry but a fatal signal is pending, handle the
+	 * If we need to retry but a signal is pending, try to handle the
 	 * signal first. We do not need to release the mmap_sem because it
 	 * would already be released in __lock_page_or_retry in mm/filemap.c.
 	 */
-	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(tsk))
+	if ((fault & VM_FAULT_RETRY) &&
+	    fault_should_check_signal(user_mode(regs)))
 		return;

Quoted 5 lines

arch/riscv/mm/fault.c asmlinkage void do_page_fault(struct pt_regs *regs) · line 122
Quoted 1 line
arch/riscv/mm/fault.c +0 -0
 	if (unlikely(fault & VM_FAULT_ERROR)) {
Acked-by Palmer Dabbelt <[email protected]> # RISC-V parts
I'm assuming this is going in through some other tree.

Hi, Palmer,

Thanks for reviewing!

There's a new version here, please feel free to have a look too:

https://lore.kernel.org/lkml/[email protected]/

Regards,

Signature · 1 line

Peter Xu

Peter Xu

author reviewer

2019-09-23 04:26 UTC #15 lore raw

PATCH v4 6/10 userfaultfd: Don't retake mmap_sem to emulate NOPAGE

In reply to Peter Xu

This patch removes the risk path in handle_userfault() then we will be sure that the callers of handle_mm_fault() will know that the VMAs might have changed. Meanwhile with previous patch we don't lose responsiveness as well since the core mm code now can handle the nonfatal userspace signals even if we return VM_FAULT_RETRY.

Reviewed-by Jerome Glisse <[email protected]>

Signed-off-by Peter Xu <[email protected]>

---
 fs/userfaultfd.c | 24 ------------------------
 1 file changed, 24 deletions(-)

fs/userfaultfd.c +0 ~~-24~~

index 3c55ee64bcb1..2b3b48e94ae4 100644
@@ -522,30 +522,6 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
 	__set_current_state(TASK_RUNNING);
 
-	if (return_to_userland) {
-		if (signal_pending(current) &&
-		    !fatal_signal_pending(current)) {
-			/*
-			 * If we got a SIGSTOP or SIGCONT and this is
-			 * a normal userland page fault, just let
-			 * userland return so the signal will be
-			 * handled and gdb debugging works.  The page
-			 * fault code immediately after we return from
-			 * this function is going to release the
-			 * mmap_sem and it's not depending on it
-			 * (unlike gup would if we were not to return
-			 * VM_FAULT_RETRY).
-			 *
-			 * If a fatal signal is pending we still take
-			 * the streamlined VM_FAULT_RETRY failure path
-			 * and there's no need to retake the mmap_sem
-			 * in such case.
-			 */
-			down_read(&mm->mmap_sem);
-			ret = VM_FAULT_NOPAGE;
-		}
-	}
-
 	/*
 	 * Here we race with the list_del; list_add in
 	 * userfaultfd_ctx_read(), however because we don't ever run

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:26 UTC #16 lore raw

PATCH v4 7/10 mm: Allow VM_FAULT_RETRY for multiple times

In reply to Peter Xu

The idea comes from a discussion between Linus and Andrea [1].

Before this patch we only allow a page fault to retry once. We achieved this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing handle_mm_fault() the second time. This was majorly used to avoid unexpected starvation of the system by looping over forever to handle the page fault on a single page. However that should hardly happen, and after all for each code path to return a VM_FAULT_RETRY we'll first wait for a condition (during which time we should possibly yield the cpu) to happen before VM_FAULT_RETRY is really returned.

This patch removes the restriction by keeping the FAULT_FLAG_ALLOW_RETRY flag when we receive VM_FAULT_RETRY. It means that the page fault handler now can retry the page fault for multiple times if necessary without the need to generate another page fault event. Meanwhile we still keep the FAULT_FLAG_TRIED flag so page fault handler can still identify whether a page fault is the first attempt or not.

Then we'll have these combinations of fault flags (only considering ALLOW_RETRY flag and TRIED flag):

- ALLOW_RETRY and !TRIED: this means the page fault allows to retry, and this is the first try

- ALLOW_RETRY and TRIED: this means the page fault allows to
retry, and this is not the first try

- !ALLOW_RETRY and !TRIED: this means the page fault does not allow to retry at all

- !ALLOW_RETRY and TRIED: this is forbidden and should never be used

In existing code we have multiple places that has taken special care of the first condition above by checking against (fault_flags & FAULT_FLAG_ALLOW_RETRY). This patch introduces a simple helper to detect the first retry of a page fault by checking against both (fault_flags & FAULT_FLAG_ALLOW_RETRY) and !(fault_flag & FAULT_FLAG_TRIED) because now even the 2nd try will have the ALLOW_RETRY set, then use that helper in all existing special paths. One example is in __lock_page_or_retry(), now we'll drop the mmap_sem only in the first attempt of page fault and we'll keep it in follow up retries, so old locking behavior will be retained.

This will be a nice enhancement for current code [2] at the same time a supporting material for the future userfaultfd-writeprotect work, since in that work there will always be an explicit userfault writeprotect retry for protected pages, and if that cannot resolve the page fault (e.g., when userfaultfd-writeprotect is used in conjunction with swapped pages) then we'll possibly need a 3rd retry of the page fault. It might also benefit other potential users who will have similar requirement like userfault write-protection.

GUP code is not touched yet and will be covered in follow up patch.

Please read the thread below for more information.

[1] https://lore.kernel.org/lkml/[email protected]/
[2] https://lore.kernel.org/lkml/[email protected]/

Reviewed-by Jerome Glisse <[email protected]>

Signed-off-by Peter Xu <[email protected]>

---
 arch/alpha/mm/fault.c           |  2 +-
 arch/arc/mm/fault.c             |  1 -
 arch/arm/mm/fault.c             |  3 ---
 arch/arm64/mm/fault.c           |  5 -----
 arch/hexagon/mm/vm_fault.c      |  1 -
 arch/ia64/mm/fault.c            |  1 -
 arch/m68k/mm/fault.c            |  3 ---
 arch/microblaze/mm/fault.c      |  1 -
 arch/mips/mm/fault.c            |  1 -
 arch/nds32/mm/fault.c           |  1 -
 arch/nios2/mm/fault.c           |  3 ---
 arch/openrisc/mm/fault.c        |  1 -
 arch/parisc/mm/fault.c          |  4 +---
 arch/powerpc/mm/fault.c         |  6 ------
 arch/riscv/mm/fault.c           |  5 -----
 arch/s390/mm/fault.c            |  5 +----
 arch/sh/mm/fault.c              |  1 -
 arch/sparc/mm/fault_32.c        |  1 -
 arch/sparc/mm/fault_64.c        |  1 -
 arch/um/kernel/trap.c           |  1 -
 arch/unicore32/mm/fault.c       |  4 +---
 arch/x86/mm/fault.c             |  2 --
 arch/xtensa/mm/fault.c          |  1 -
 drivers/gpu/drm/ttm/ttm_bo_vm.c | 12 ++++++++---
 include/linux/mm.h              | 37 +++++++++++++++++++++++++++++++++
 mm/filemap.c                    |  2 +-
 mm/shmem.c                      |  2 +-
 27 files changed, 52 insertions(+), 55 deletions(-)

arch/alpha/mm/fault.c +1 -1

index ab1d4212d658..e032d2d03012 100644
@@ -170,7 +170,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would
 			 * have already released it in __lock_page_or_retry

arch/arc/mm/fault.c +0 -1

index 27adf4e608e4..bbcde83e010a 100644
@@ -151,7 +151,6 @@ void do_page_fault(unsigned long address, struct pt_regs *regs)
 		 * retry state machine
 		 */
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}

arch/arm/mm/fault.c +0 -3

index f00fb4eafe54..5f1fb46a37b0 100644
@@ -320,9 +320,6 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 					regs, addr);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			* of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}

arch/arm64/mm/fault.c +0 -5

index 0d3fe0ea6a70..8c26097bcf0d 100644
@@ -491,12 +491,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
 				return 0;
 		}
 
-		/*
-		 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk of
-		 * starvation.
-		 */
 		if (mm_flags & FAULT_FLAG_ALLOW_RETRY) {
-			mm_flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			mm_flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}

arch/hexagon/mm/vm_fault.c +0 -1

index 88a2e5635bfb..a299d2142cbb 100644
@@ -103,7 +103,6 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 			else
 				current->min_flt++;
 			if (fault & VM_FAULT_RETRY) {
-				flags &= ~FAULT_FLAG_ALLOW_RETRY;
 				flags |= FAULT_FLAG_TRIED;
 				goto retry;
 			}

arch/ia64/mm/fault.c +0 -1

index 8d47acf50fda..7679e960c685 100644
@@ -168,7 +168,6 @@ ia64_do_page_fault (unsigned long address, unsigned long isr, struct pt_regs *re
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would

arch/m68k/mm/fault.c +0 -3

index 103f93ba8139..d4ef4fdf4de4 100644
@@ -163,9 +163,6 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*

arch/microblaze/mm/fault.c +0 -1

index 8b0615eab4b6..9a359568f70a 100644
@@ -237,7 +237,6 @@ void do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*

arch/mips/mm/fault.c +0 -1

index 48aac20a1ded..5eeea572ff3f 100644
@@ -179,7 +179,6 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 			tsk->min_flt++;
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*

arch/nds32/mm/fault.c +0 -1

index baa44f9d0b4a..954ca83d3289 100644
@@ -243,7 +243,6 @@ void do_page_fault(unsigned long entry, unsigned long addr,
 				      1, regs, addr);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would

arch/nios2/mm/fault.c +0 -3

index f9f178484184..07f467577e77 100644
@@ -158,9 +158,6 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long cause,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*

arch/openrisc/mm/fault.c +0 -1

index 8ba3696dd10c..e7dadbdb21b3 100644
@@ -182,7 +182,6 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long address,
 		else
 			tsk->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would

arch/parisc/mm/fault.c +1 -3

index 163dcb080c7b..c837da780a79 100644
@@ -329,14 +329,12 @@ void do_page_fault(struct pt_regs *regs, unsigned long code,
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
-
 			/*
 			 * No need to up_read(&mm->mmap_sem) as we would
 			 * have already released it in __lock_page_or_retry
 			 * in mm/filemap.c.
 			 */
-
+			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
 	}

arch/powerpc/mm/fault.c +0 -6

index d321a6c5fe62..321f24d0762f 100644
@@ -588,13 +588,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 	 * case.
 	 */
 	if (unlikely(fault & VM_FAULT_RETRY)) {
-		/* We retry only once */
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
-			/*
-			 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation.
-			 */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			if (is_user && signal_pending(current))
 				return 0;

arch/riscv/mm/fault.c +0 -5

index ea8f301de65b..d1710ef75432 100644
@@ -143,11 +143,6 @@ asmlinkage void do_page_fault(struct pt_regs *regs)
 				      1, regs, addr);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			/*
-			 * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation.
-			 */
-			flags &= ~(FAULT_FLAG_ALLOW_RETRY);
 			flags |= FAULT_FLAG_TRIED;
 
 			/*

arch/s390/mm/fault.c +1 -4

index 3ad77501deef..46ef1159d146 100644
@@ -514,10 +514,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
 				fault = VM_FAULT_PFAULT;
 				goto out_up;
 			}
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			 * of starvation. */
-			flags &= ~(FAULT_FLAG_ALLOW_RETRY |
-				   FAULT_FLAG_RETRY_NOWAIT);
+			flags &= ~FAULT_FLAG_RETRY_NOWAIT;
 			flags |= FAULT_FLAG_TRIED;
 			down_read(&mm->mmap_sem);
 			goto retry;

arch/sh/mm/fault.c +0 -1

index f620282a37fd..2e9cf3fd395f 100644
@@ -481,7 +481,6 @@ asmlinkage void __kprobes do_page_fault(struct pt_regs *regs,
 				      regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/*

arch/sparc/mm/fault_32.c +0 -1

index 9af0c3ad50d6..97494086f1e5 100644
@@ -261,7 +261,6 @@ asmlinkage void do_sparc_fault(struct pt_regs *regs, int text_fault, int write,
 				      1, regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would

arch/sparc/mm/fault_64.c +0 -1

index 566f05f9040b..a1730c3a8f30 100644
@@ -446,7 +446,6 @@ asmlinkage void __kprobes do_sparc64_fault(struct pt_regs *regs)
 				      1, regs, address);
 		}
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			/* No need to up_read(&mm->mmap_sem) as we would

arch/um/kernel/trap.c +0 -1

index 3c72111f27e9..063da0930d31 100644
@@ -98,7 +98,6 @@ int handle_page_fault(unsigned long address, unsigned long ip,
 			else
 				current->min_flt++;
 			if (fault & VM_FAULT_RETRY) {
-				flags &= ~FAULT_FLAG_ALLOW_RETRY;
 				flags |= FAULT_FLAG_TRIED;
 
 				goto retry;

arch/unicore32/mm/fault.c +1 -3

index 04c193439c97..8b3367ec0d80 100644
@@ -260,9 +260,7 @@ static int do_pf(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 		else
 			tsk->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-			* of starvation. */
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}
 	}

arch/x86/mm/fault.c +0 -2

index f7836472961e..7664f0f89ef6 100644
@@ -1447,9 +1447,7 @@ void do_user_addr_fault(struct pt_regs *regs,
 	 * that we made any progress. Handle this case first.
 	 */
 	if (unlikely(fault & VM_FAULT_RETRY)) {
-		/* Retry at most once */
 		if (flags & FAULT_FLAG_ALLOW_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 			if ((flags & FAULT_FLAG_USER) && signal_pending(tsk))
 				return;

arch/xtensa/mm/fault.c +0 -1

index 094606676c36..1d91a23d27d3 100644
@@ -129,7 +129,6 @@ void do_page_fault(struct pt_regs *regs)
 		else
 			current->min_flt++;
 		if (fault & VM_FAULT_RETRY) {
-			flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			flags |= FAULT_FLAG_TRIED;
 
 			 /* No need to up_read(&mm->mmap_sem) as we would

drivers/gpu/drm/ttm/ttm_bo_vm.c +9 -3

index 6dacff49c1cc..8f2f9ee6effa 100644
@@ -61,9 +61,10 @@ static vm_fault_t ttm_bo_vm_fault_idle(struct ttm_buffer_object *bo,
 
 	/*
 	 * If possible, avoid waiting for GPU with mmap_sem
-	 * held.
+	 * held.  We only do this if the fault allows retry and this
+	 * is the first attempt.
 	 */
-	if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
+	if (fault_flag_allow_retry_first(vmf->flags)) {
 		ret = VM_FAULT_RETRY;
 		if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
 			goto out_unlock;

@@ -132,7 +133,12 @@ static vm_fault_t ttm_bo_vm_fault(struct vm_fault *vmf)
 	 * for the buffer to become unreserved.
 	 */
 	if (unlikely(!reservation_object_trylock(bo->resv))) {
-		if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
+		/*
+		 * If the fault allows retry and this is the first
+		 * fault attempt, we try to release the mmap_sem
+		 * before waiting
+		 */
+		if (fault_flag_allow_retry_first(vmf->flags)) {
 			if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
 				ttm_bo_get(bo);
 				up_read(&vmf->vma->vm_mm->mmap_sem);

include/linux/mm.h +37 -0

index 53ec7abb8472..0fdbdcb257d6 100644
@@ -396,6 +396,25 @@ extern pgprot_t protection_map[16];
  * @FAULT_FLAG_REMOTE: The fault is not for current task/mm.
  * @FAULT_FLAG_INSTRUCTION: The fault was during an instruction fetch.
  * @FAULT_FLAG_INTERRUPTIBLE: The fault can be interrupted by non-fatal signals.
+ *
+ * About @FAULT_FLAG_ALLOW_RETRY and @FAULT_FLAG_TRIED: we can specify
+ * whether we would allow page faults to retry by specifying these two
+ * fault flags correctly.  Currently there can be three legal combinations:
+ *
+ * (a) ALLOW_RETRY and !TRIED:  this means the page fault allows retry, and
+ *                              this is the first try
+ *
+ * (b) ALLOW_RETRY and TRIED:   this means the page fault allows retry, and
+ *                              we've already tried at least once
+ *
+ * (c) !ALLOW_RETRY and !TRIED: this means the page fault does not allow retry
+ *
+ * The unlisted combination (!ALLOW_RETRY && TRIED) is illegal and should never
+ * be used.  Note that page faults can be allowed to retry for multiple times,
+ * in which case we'll have an initial fault with flags (a) then later on
+ * continuous faults with flags (b).  We should always try to detect pending
+ * signals before a retry to make sure the continuous page faults can still be
+ * interrupted if necessary.
  */
 #define FAULT_FLAG_WRITE			0x01
 #define FAULT_FLAG_MKWRITE			0x02

@@ -416,6 +435,24 @@ extern pgprot_t protection_map[16];
 			     FAULT_FLAG_KILLABLE | \
 			     FAULT_FLAG_INTERRUPTIBLE)
 
+/**
+ * fault_flag_allow_retry_first - check ALLOW_RETRY the first time
+ *
+ * This is mostly used for places where we want to try to avoid taking
+ * the mmap_sem for too long a time when waiting for another condition
+ * to change, in which case we can try to be polite to release the
+ * mmap_sem in the first round to avoid potential starvation of other
+ * processes that would also want the mmap_sem.
+ *
+ * Return: true if the page fault allows retry and this is the first
+ * attempt of the fault handling; false otherwise.
+ */
+static inline bool fault_flag_allow_retry_first(unsigned int flags)
+{
+	return (flags & FAULT_FLAG_ALLOW_RETRY) &&
+	    (!(flags & FAULT_FLAG_TRIED));
+}
+
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
 	{ FAULT_FLAG_MKWRITE,		"MKWRITE" }, \

mm/filemap.c +1 -1

index d0cf700bf201..543404617f5a 100644
@@ -1399,7 +1399,7 @@ EXPORT_SYMBOL_GPL(__lock_page_killable);
 int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
 			 unsigned int flags)
 {
-	if (flags & FAULT_FLAG_ALLOW_RETRY) {
+	if (fault_flag_allow_retry_first(flags)) {
 		/*
 		 * CAUTION! In this case, mmap_sem is not released
 		 * even though return 0.

mm/shmem.c +1 -1

index 2bed4761f279..1af8d8e60231 100644
@@ -2011,7 +2011,7 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
 			DEFINE_WAIT_FUNC(shmem_fault_wait, synchronous_wake_function);
 
 			ret = VM_FAULT_NOPAGE;
-			if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
+			if (fault_flag_allow_retry_first(vmf->flags) &&
 			   !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
 				/* It's polite to up mmap_sem if we can */
 				up_read(&vma->vm_mm->mmap_sem);

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:26 UTC #17 lore raw

PATCH v4 8/10 mm/gup: Allow VM_FAULT_RETRY for multiple times

In reply to Peter Xu

This is the gup counterpart of the change that allows the VM_FAULT_RETRY to happen for more than once. One thing to mention is that we must check the fatal signal here before retry because the GUP can be interrupted by that, otherwise we can loop forever.

Signed-off-by Peter Xu <[email protected]>

---
 mm/gup.c     | 27 +++++++++++++++++++++------
 mm/hugetlb.c |  6 ++++--
 2 files changed, 25 insertions(+), 8 deletions(-)

mm/gup.c +21 -6

index e60d32f1674d..d2811bb15a25 100644
@@ -644,7 +644,10 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
 	if (*flags & FOLL_TRIED) {
-		VM_WARN_ON_ONCE(fault_flags & FAULT_FLAG_ALLOW_RETRY);
+		/*
+		 * Note: FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_TRIED
+		 * can co-exist
+		 */
 		fault_flags |= FAULT_FLAG_TRIED;
 	}

@@ -994,7 +997,6 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 		down_read(&mm->mmap_sem);
 		if (!(fault_flags & FAULT_FLAG_TRIED)) {
 			*unlocked = true;
-			fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
 			fault_flags |= FAULT_FLAG_TRIED;
 			goto retry;
 		}

@@ -1069,17 +1071,30 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 		if (likely(pages))
 			pages += ret;
 		start += ret << PAGE_SHIFT;
+		lock_dropped = true;
 
+retry:
 		/*
 		 * Repeat on the address that fired VM_FAULT_RETRY
-		 * without FAULT_FLAG_ALLOW_RETRY but with
-		 * FAULT_FLAG_TRIED.
+		 * with both FAULT_FLAG_ALLOW_RETRY and
+		 * FAULT_FLAG_TRIED.  Note that GUP can be interrupted
+		 * by fatal signals, so we need to check it before we
+		 * start trying again otherwise it can loop forever.
 		 */
+
+		if (fatal_signal_pending(current))
+			break;
+
 		*locked = 1;
-		lock_dropped = true;
 		down_read(&mm->mmap_sem);
+
 		ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
-				       pages, NULL, NULL);
+				       pages, NULL, locked);
+		if (!*locked) {
+			/* Continue to retry until we succeeded */
+			BUG_ON(ret != 0);
+			goto retry;
+		}
 		if (ret != 1) {
 			BUG_ON(ret > 1);
 			if (!pages_done)

mm/hugetlb.c +4 -2

index 31c2a6275023..d0c98cff5b0f 100644
@@ -4347,8 +4347,10 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
 					FAULT_FLAG_RETRY_NOWAIT;
 			if (flags & FOLL_TRIED) {
-				VM_WARN_ON_ONCE(fault_flags &
-						FAULT_FLAG_ALLOW_RETRY);
+				/*
+				 * Note: FAULT_FLAG_ALLOW_RETRY and
+				 * FAULT_FLAG_TRIED can co-exist
+				 */
 				fault_flags |= FAULT_FLAG_TRIED;
 			}
 			ret = hugetlb_fault(mm, vma, vaddr, fault_flags);

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:26 UTC #18 lore raw

PATCH v4 9/10 mm/gup: Allow to react to fatal signals

In reply to Peter Xu

The existing gup code does not react to the fatal signals in many code paths. For example, in one retry path of gup we're still using down_read() rather than down_read_killable(). Also, when doing page faults we don't pass in FAULT_FLAG_KILLABLE as well, which means that within the faulting process we'll wait in non-killable way as well. These were spotted by Linus during the code review of some other patches.

Let's allow the gup code to react to fatal signals to improve the responsiveness of threads when during gup and being killed.

Signed-off-by Peter Xu <[email protected]>

---
 mm/gup.c     | 12 +++++++++---
 mm/hugetlb.c |  3 ++-
 2 files changed, 11 insertions(+), 4 deletions(-)

mm/gup.c +9 -3

index d2811bb15a25..4c638473db83 100644
@@ -640,7 +640,7 @@ static int faultin_page(struct task_struct *tsk, struct vm_area_struct *vma,
 	if (*flags & FOLL_REMOTE)
 		fault_flags |= FAULT_FLAG_REMOTE;
 	if (locked)
-		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
+		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
 	if (*flags & FOLL_NOWAIT)
 		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
 	if (*flags & FOLL_TRIED) {

@@ -973,7 +973,7 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
 	vm_fault_t ret, major = 0;
 
 	if (unlocked)
-		fault_flags |= FAULT_FLAG_ALLOW_RETRY;
+		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
 
 retry:
 	vma = find_extend_vma(mm, address);

@@ -1086,7 +1086,13 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 			break;
 
 		*locked = 1;
-		down_read(&mm->mmap_sem);
+		ret = down_read_killable(&mm->mmap_sem);
+		if (ret) {
+			BUG_ON(ret > 0);
+			if (!pages_done)
+				pages_done = ret;
+			break;
+		}
 
 		ret = __get_user_pages(tsk, mm, start, 1, flags | FOLL_TRIED,
 				       pages, NULL, locked);

mm/hugetlb.c +2 -1

index d0c98cff5b0f..84034154d50e 100644
@@ -4342,7 +4342,8 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			if (flags & FOLL_WRITE)
 				fault_flags |= FAULT_FLAG_WRITE;
 			if (locked)
-				fault_flags |= FAULT_FLAG_ALLOW_RETRY;
+				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
+					FAULT_FLAG_KILLABLE;
 			if (flags & FOLL_NOWAIT)
 				fault_flags |= FAULT_FLAG_ALLOW_RETRY |
 					FAULT_FLAG_RETRY_NOWAIT;

Signature · 1 line

2.21.0

Peter Xu

author reviewer

2019-09-23 04:26 UTC #19 lore raw

PATCH v4 10/10 mm/userfaultfd: Honor FAULT_FLAG_KILLABLE in fault path

In reply to Peter Xu

Userfaultfd fault path was by default killable even if the caller does not have FAULT_FLAG_KILLABLE. That makes sense before in that when with gup we don't have FAULT_FLAG_KILLABLE properly set before. Now after previous patch we've got FAULT_FLAG_KILLABLE applied even for gup code so it should also make sense to let userfaultfd to honor the FAULT_FLAG_KILLABLE.

Because we're unconditionally setting FAULT_FLAG_KILLABLE in gup code right now, this patch should have no functional change. It also cleaned the code a little bit by introducing some helpers.

Signed-off-by Peter Xu <[email protected]>

---
 fs/userfaultfd.c | 36 ++++++++++++++++++++++++++++--------
 1 file changed, 28 insertions(+), 8 deletions(-)

fs/userfaultfd.c +28 -8

index 2b3b48e94ae4..8c5863ccbf0e 100644
@@ -334,6 +334,30 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 	return ret;
 }
 
+/* Should pair with userfaultfd_signal_pending() */
+static inline long userfaultfd_get_blocking_state(unsigned int flags)
+{
+	if (flags & FAULT_FLAG_INTERRUPTIBLE)
+		return TASK_INTERRUPTIBLE;
+
+	if (flags & FAULT_FLAG_KILLABLE)
+		return TASK_KILLABLE;
+
+	return TASK_UNINTERRUPTIBLE;
+}
+
+/* Should pair with userfaultfd_get_blocking_state() */
+static inline bool userfaultfd_signal_pending(unsigned int flags)
+{
+	if (flags & FAULT_FLAG_INTERRUPTIBLE)
+		return signal_pending(current);
+
+	if (flags & FAULT_FLAG_KILLABLE)
+		return fatal_signal_pending(current);
+
+	return false;
+}
+
 /*
  * The locking rules involved in returning VM_FAULT_RETRY depending on
  * FAULT_FLAG_ALLOW_RETRY, FAULT_FLAG_RETRY_NOWAIT and

@@ -355,7 +379,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	struct userfaultfd_ctx *ctx;
 	struct userfaultfd_wait_queue uwq;
 	vm_fault_t ret = VM_FAULT_SIGBUS;
-	bool must_wait, return_to_userland;
+	bool must_wait;
 	long blocking_state;
 
 	/*

@@ -462,9 +486,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	uwq.ctx = ctx;
 	uwq.waken = false;
 
-	return_to_userland = vmf->flags & FAULT_FLAG_INTERRUPTIBLE;
-	blocking_state = return_to_userland ? TASK_INTERRUPTIBLE :
-			 TASK_KILLABLE;
+	blocking_state = userfaultfd_get_blocking_state(vmf->flags);
 
 	spin_lock_irq(&ctx->fault_pending_wqh.lock);
 	/*

@@ -490,8 +512,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 	up_read(&mm->mmap_sem);
 
 	if (likely(must_wait && !READ_ONCE(ctx->released) &&
-		   (return_to_userland ? !signal_pending(current) :
-		    !fatal_signal_pending(current)))) {
+		   userfaultfd_signal_pending(vmf->flags))) {
 		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
 		schedule();
 		ret |= VM_FAULT_MAJOR;

@@ -513,8 +534,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 			set_current_state(blocking_state);
 			if (READ_ONCE(uwq.waken) ||
 			    READ_ONCE(ctx->released) ||
-			    (return_to_userland ? signal_pending(current) :
-			     fatal_signal_pending(current)))
+			    userfaultfd_signal_pending(vmf->flags))
 				break;
 			schedule();
 		}

Signature · 1 line

2.21.0