HWPOISON: Intro

Another version of the hwpoison patchkit. I addressed all feedback, except:
I didn't move the handlers into other files for now, prefer to keep things together for now
I'm keeping an own pagepoison bit because I think that's cleaner than any other hacks.

Andrew, please put it into mm for .31 track.

The patchkit is also available in
git://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-mce-2.6.git hwpoison

Thanks,
-Andi

---

Upcoming Intel CPUs have support for recovering from some memory errors (``MCA recovery''). This requires the OS to declare a page "poisoned", kill the processes associated with it and avoid using it in the future.

This patchkit implements the necessary infrastructure in the VM.

To quote the overview comment:

* High level machine check handler. Handles pages reported by the
* hardware as being corrupted usually due to a 2bit ECC memory or cache
* failure.
*
* This focusses on pages detected as corrupted in the background.
* When the current CPU tries to consume corruption the currently
* running process can just be killed directly instead. This implies
* that if the error cannot be handled for some reason it's safe to
* just ignore it because no corruption has been consumed yet. Instead
* when that happens another machine check will happen.
*
* Handles page cache pages in various states. The tricky part
* here is that we can access any page asynchronous to other VM
* users, because memory failures could happen anytime and anywhere,
* possibly violating some of their assumptions. This is why this code
* has to be extremely careful. Generally it tries to use normal locking
* rules, as in get the standard locks, even if that means the
* error handling takes potentially a long time.
*
* Some of the operations here are somewhat inefficient and have non
* linear algorithmic complexity, because the data structures have not
* been optimized for this case. This is in particular the case
* for the mapping from a vma to a process. Since this case is expected
* to be rare we hope we can get away with this.

The code consists of a the high level handler in mm/memory-failure.c, a new page poison bit and various checks in the VM to handle poisoned pages.

The main target right now is KVM guests, but it works for all kinds of applications.

For the KVM use there was need for a new signal type so that KVM can inject the machine check into the guest with the proper address. This in theory allows other applications to handle memory failures too. The expection is that near all applications won't do that, but some very specialized ones might.

This is not fully complete yet, in particular there are still ways to access poison through various ways (crash dump, /proc/kcore etc.) that need to be plugged too.

Also undoubtedly the high level handler still has bugs and cases it cannot recover from. For example nonlinear mappings deadlock right now and a few other cases lose references. Huge pages are not supported yet. Any additional testing, reviewing etc. welcome.

The patch series requires the earlier x86 MCE feature series for the x86 specific action optional part. The code can be tested without the x86 specific part using the injector, this only requires to enable the Kconfig entry manually in some Kconfig file (by default it is implicitely enabled by the architecture)

v2: Lots of smaller changes in the series based on review feedback. Rename Poison to HWPoison after akpm's request.
A new pfn based injector based on feedback.
A lot of improvements mostly from Fengguang Wu
See comments in the individual patches.
v3: Various updates, see changelogs in individual patches.

-Andi

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Hardware poisoned pages need special handling in the VM and shouldn't be touched again. This requires a new page flag. Define it here.

The page flags wars seem to be over, so it shouldn't be a problem to get a new one.

v2: Add TestSetHWPoison (suggested by Johannes Weiner)

---
 include/linux/page-flags.h |   17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

Index: linux/include/linux/page-flags.h
===================================================================
@@ -51,6 +51,9 @@
  * PG_buddy is set to indicate that the page is free and in the buddy system
  * (see mm/page_alloc.c).
  *
+ * PG_hwpoison indicates that a page got corrupted in hardware and contains
+ * data with incorrect ECC bits that triggered a machine check. Accessing is
+ * not safe since it may cause another machine check. Don't touch!
  */
 
 /*

@@ -104,6 +107,9 @@
 #ifdef CONFIG_IA64_UNCACHED_ALLOCATOR
 	PG_uncached,		/* Page has been mapped as uncached */
 #endif
+#ifdef CONFIG_MEMORY_FAILURE
+	PG_hwpoison,		/* hardware poisoned page. Don't touch */
+#endif
 	__NR_PAGEFLAGS,
 
 	/* Filesystems */

@@ -273,6 +279,15 @@
 PAGEFLAG_FALSE(Uncached)
 #endif
 
+#ifdef CONFIG_MEMORY_FAILURE
+PAGEFLAG(HWPoison, hwpoison)
+TESTSETFLAG(HWPoison, hwpoison)
+#define __PG_HWPOISON (1UL << PG_hwpoison)
+#else
+PAGEFLAG_FALSE(HWPoison)
+#define __PG_HWPOISON 0
+#endif
+
 static inline int PageUptodate(struct page *page)
 {
 	int ret = test_bit(PG_uptodate, &(page)->flags);

@@ -403,7 +418,7 @@
 	 1 << PG_private | 1 << PG_private_2 | \
 	 1 << PG_buddy	 | 1 << PG_writeback | 1 << PG_reserved | \
 	 1 << PG_slab	 | 1 << PG_swapcache | 1 << PG_active | \
-	 __PG_UNEVICTABLE | __PG_MLOCKED)
+	 __PG_HWPOISON  | __PG_UNEVICTABLE | __PG_MLOCKED)
 
 /*
  * Flags checked when a page is prepped for return by the page allocator.

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From: Fengguang Wu <[email protected]>

Export the new poison flag in /proc/kpageflags. Poisoned pages are moderately interesting even for administrators, so export them here. Also useful for debugging.

AK: I extracted this out of a larger patch from Fengguang Wu.

---
 fs/proc/page.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux/fs/proc/page.c
===================================================================
@@ -79,6 +79,7 @@
 #define KPF_WRITEBACK  8
 #define KPF_RECLAIM    9
 #define KPF_BUDDY     10
+#define KPF_HWPOISON  11
 
 #define kpf_copy_bit(flags, dstpos, srcpos) (((flags >> srcpos) & 1) << dstpos)
 

@@ -118,6 +119,9 @@
 			kpf_copy_bit(kflags, KPF_WRITEBACK, PG_writeback) |
 			kpf_copy_bit(kflags, KPF_RECLAIM, PG_reclaim) |
 			kpf_copy_bit(kflags, KPF_BUDDY, PG_buddy);
+#ifdef CONFIG_MEMORY_FAILURE
+		uflags |= kpf_copy_bit(kflags, KPF_HWPOISON, PG_hwpoison);
+#endif
 
 		if (put_user(uflags, out++)) {
 			ret = -EFAULT;

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Needed for later patch that walks rmap entries on its own.

This used to be very frowned upon, but memory-failure.c does some rather specialized rmap walking and rmap has been stable for quite some time, so I think it's ok now to export it.

---
 include/linux/rmap.h |    6 ++++++
 mm/rmap.c            |    4 ++--
 2 files changed, 8 insertions(+), 2 deletions(-)

Index: linux/include/linux/rmap.h
===================================================================
@@ -118,6 +118,12 @@
 }
 #endif
 
+/*
+ * Called by memory-failure.c to kill processes.
+ */
+struct anon_vma *page_lock_anon_vma(struct page *page);
+void page_unlock_anon_vma(struct anon_vma *anon_vma);
+
 #else	/* !CONFIG_MMU */
 
 #define anon_vma_init()		do {} while (0)

Index: linux/mm/rmap.c
===================================================================
@@ -191,7 +191,7 @@
  * Getting a lock on a stable anon_vma from a page off the LRU is
  * tricky: page_lock_anon_vma rely on RCU to guard against the races.
  */
-static struct anon_vma *page_lock_anon_vma(struct page *page)
+struct anon_vma *page_lock_anon_vma(struct page *page)
 {
 	struct anon_vma *anon_vma;
 	unsigned long anon_mapping;

@@ -211,7 +211,7 @@
 	return NULL;
 }
 
-static void page_unlock_anon_vma(struct anon_vma *anon_vma)
+void page_unlock_anon_vma(struct anon_vma *anon_vma)
 {
 	spin_unlock(&anon_vma->lock);
 	rcu_read_unlock();

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Memory migration uses special swap entry types to trigger special actions on page faults. Extend this mechanism to also support poisoned swap entries, to trigger poison handling on page faults. This allows follow-on patches to prevent processes from faulting in poisoned pages again.

v2: Fix overflow in MAX_SWAPFILES (Fengguang Wu)
v3: Better overflow fix (Hidehiro Kawai)

---
 include/linux/swap.h    |   34 ++++++++++++++++++++++++++++------
 include/linux/swapops.h |   38 ++++++++++++++++++++++++++++++++++++++
 mm/swapfile.c           |    4 ++--
 3 files changed, 68 insertions(+), 8 deletions(-)

Index: linux/include/linux/swap.h
===================================================================
@@ -34,16 +34,38 @@
  * the type/offset into the pte as 5/27 as well.
  */
 #define MAX_SWAPFILES_SHIFT	5
-#ifndef CONFIG_MIGRATION
-#define MAX_SWAPFILES		(1 << MAX_SWAPFILES_SHIFT)
+
+/*
+ * Use some of the swap files numbers for other purposes. This
+ * is a convenient way to hook into the VM to trigger special
+ * actions on faults.
+ */
+
+/*
+ * NUMA node memory migration support
+ */
+#ifdef CONFIG_MIGRATION
+#define SWP_MIGRATION_NUM 2
+#define SWP_MIGRATION_READ	(MAX_SWAPFILES + SWP_HWPOISON_NUM)
+#define SWP_MIGRATION_WRITE	(MAX_SWAPFILES + SWP_HWPOISON_NUM + 1)
 #else
-/* Use last two entries for page migration swap entries */
-#define MAX_SWAPFILES		((1 << MAX_SWAPFILES_SHIFT)-2)
-#define SWP_MIGRATION_READ	MAX_SWAPFILES
-#define SWP_MIGRATION_WRITE	(MAX_SWAPFILES + 1)
+#define SWP_MIGRATION_NUM 0
 #endif
 
 /*
+ * Handling of hardware poisoned pages with memory corruption.
+ */
+#ifdef CONFIG_MEMORY_FAILURE
+#define SWP_HWPOISON_NUM 1
+#define SWP_HWPOISON		MAX_SWAPFILES
+#else
+#define SWP_HWPOISON_NUM 0
+#endif
+
+#define MAX_SWAPFILES \
+	((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
+
+/*
  * Magic header for a swap area. The first part of the union is
  * what the swap magic looks like for the old (limited to 128MB)
  * swap area format, the second part of the union adds - in the

Index: linux/include/linux/swapops.h
===================================================================
@@ -131,3 +131,41 @@
 
 #endif
 
+#ifdef CONFIG_MEMORY_FAILURE
+/*
+ * Support for hardware poisoned pages
+ */
+static inline swp_entry_t make_hwpoison_entry(struct page *page)
+{
+	BUG_ON(!PageLocked(page));
+	return swp_entry(SWP_HWPOISON, page_to_pfn(page));
+}
+
+static inline int is_hwpoison_entry(swp_entry_t entry)
+{
+	return swp_type(entry) == SWP_HWPOISON;
+}
+#else
+
+static inline swp_entry_t make_hwpoison_entry(struct page *page)
+{
+	return swp_entry(0, 0);
+}
+
+static inline int is_hwpoison_entry(swp_entry_t swp)
+{
+	return 0;
+}
+#endif
+
+#if defined(CONFIG_MEMORY_FAILURE) || defined(CONFIG_MIGRATION)
+static inline int non_swap_entry(swp_entry_t entry)
+{
+	return swp_type(entry) >= MAX_SWAPFILES;
+}
+#else
+static inline int non_swap_entry(swp_entry_t entry)
+{
+	return 0;
+}
+#endif

Index: linux/mm/swapfile.c
===================================================================
@@ -579,7 +579,7 @@
 	struct swap_info_struct *p;
 	struct page *page = NULL;
 
-	if (is_migration_entry(entry))
+	if (non_swap_entry(entry))
 		return 1;
 
 	p = swap_info_get(entry);

@@ -1949,7 +1949,7 @@
 	unsigned long offset, type;
 	int result = 0;
 
-	if (is_migration_entry(entry))
+	if (non_swap_entry(entry))
 		return 1;
 
 	type = swp_type(entry);

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Add new SIGBUS codes for reporting machine checks as signals. When the hardware detects an uncorrected ECC error it can trigger these signals.

This is needed for telling KVM's qemu about machine checks that happen to guests, so that it can inject them, but might be also useful for other programs. I find it useful in my test programs.

This patch merely defines the new types.

- Define two new si_codes for SIGBUS. BUS_MCEERR_AO and BUS_MCEERR_AR
* BUS_MCEERR_AO is for "Action Optional" machine checks, which means that some corruption has been detected in the background, but nothing has been consumed so far. The program can ignore those if it wants (but most programs would already get killed)
* BUS_MCEERR_AR is for "Action Required" machine checks. This happens when corrupted data is consumed or the application ran into an area which has been known to be corrupted earlier. These require immediate action and cannot just returned to. Most programs would kill themselves.
- They report the address of the corruption in the user address space in si_addr.
- Define a new si_addr_lsb field that reports the extent of the corruption to user space. That's currently always a (small) page. The user application cannot tell where in this page the corruption happened.

AK: I plan to write a man page update before anyone asks.

---
 include/asm-generic/siginfo.h |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

Index: linux/include/asm-generic/siginfo.h
===================================================================
@@ -82,6 +82,7 @@
 #ifdef __ARCH_SI_TRAPNO
 			int _trapno;	/* TRAP # which caused the signal */
 #endif
+			short _addr_lsb; /* LSB of the reported address */
 		} _sigfault;
 
 		/* SIGPOLL */

@@ -112,6 +113,7 @@
 #ifdef __ARCH_SI_TRAPNO
 #define si_trapno	_sifields._sigfault._trapno
 #endif
+#define si_addr_lsb	_sifields._sigfault._addr_lsb
 #define si_band		_sifields._sigpoll._band
 #define si_fd		_sifields._sigpoll._fd
 

@@ -192,7 +194,11 @@
 #define BUS_ADRALN	(__SI_FAULT|1)	/* invalid address alignment */
 #define BUS_ADRERR	(__SI_FAULT|2)	/* non-existant physical address */
 #define BUS_OBJERR	(__SI_FAULT|3)	/* object specific hardware error */
-#define NSIGBUS		3
+/* hardware memory error consumed on a machine check: action required */
+#define BUS_MCEERR_AR	(__SI_FAULT|4)
+/* hardware memory error detected in process but not consumed: action optional*/
+#define BUS_MCEERR_AO	(__SI_FAULT|5)
+#define NSIGBUS		5
 
 /*
  * SIGTRAP si_codes

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- Add a new VM_FAULT_HWPOISON error code to handle_mm_fault. Right now architectures have to explicitely enable poison page support, so this is forward compatible to all architectures. They only need to add it when they enable poison page support.
- Add poison page handling in swap in fault code

v2: Add missing delayacct_clear_flag (Hidehiro Kawai)
v3: Really use delayacct_clear_flag (Hidehiro Kawai)

---
 include/linux/mm.h |    3 ++-
 mm/memory.c        |   18 +++++++++++++++---
 2 files changed, 17 insertions(+), 4 deletions(-)

Index: linux/mm/memory.c
===================================================================
@@ -1315,7 +1315,8 @@
 				if (ret & VM_FAULT_ERROR) {
 					if (ret & VM_FAULT_OOM)
 						return i ? i : -ENOMEM;
-					else if (ret & VM_FAULT_SIGBUS)
+					if (ret &
+					    (VM_FAULT_HWPOISON|VM_FAULT_SIGBUS))
 						return i ? i : -EFAULT;
 					BUG();
 				}

@@ -2459,8 +2460,15 @@
 		goto out;
 
 	entry = pte_to_swp_entry(orig_pte);
-	if (is_migration_entry(entry)) {
-		migration_entry_wait(mm, pmd, address);
+	if (unlikely(non_swap_entry(entry))) {
+		if (is_migration_entry(entry)) {
+			migration_entry_wait(mm, pmd, address);
+		} else if (is_hwpoison_entry(entry)) {
+			ret = VM_FAULT_HWPOISON;
+		} else {
+			print_bad_pte(vma, address, pte, NULL);
+			ret = VM_FAULT_OOM;
+		}
 		goto out;
 	}
 	delayacct_set_flag(DELAYACCT_PF_SWAPIN);

@@ -2484,6 +2492,10 @@
 		/* Had to read the page from swap area: Major fault */
 		ret = VM_FAULT_MAJOR;
 		count_vm_event(PGMAJFAULT);
+	} else if (PageHWPoison(page)) {
+		ret = VM_FAULT_HWPOISON;
+		delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
+		goto out;
 	}
 
 	lock_page(page);

Index: linux/include/linux/mm.h
===================================================================
@@ -702,11 +702,12 @@
 #define VM_FAULT_SIGBUS	0x0002
 #define VM_FAULT_MAJOR	0x0004
 #define VM_FAULT_WRITE	0x0008	/* Special case for get_user_pages */
+#define VM_FAULT_HWPOISON 0x0010	/* Hit poisoned page */
 
 #define VM_FAULT_NOPAGE	0x0100	/* ->fault installed the pte, not return page */
 #define VM_FAULT_LOCKED	0x0200	/* ->fault locked the returned page */
 
-#define VM_FAULT_ERROR	(VM_FAULT_OOM | VM_FAULT_SIGBUS)
+#define VM_FAULT_ERROR	(VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_HWPOISON)
 
 /*
  * Can be called by the pagefault handler when it gets a VM_FAULT_OOM.

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Bail out early when hardware poisoned pages are found in page fault handling. Since they are poisoned they should not be mapped freshly into processes, because that would cause another (potentially deadly) machine check

This is generally handled in the same way as OOM, just a different error code is returned to the architecture code.

---
 mm/memory.c |    3 +++
 1 file changed, 3 insertions(+)

Index: linux/mm/memory.c
===================================================================
@@ -2659,6 +2659,9 @@
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
 		return ret;
 
+	if (unlikely(PageHWPoison(vmf.page)))
+		return VM_FAULT_HWPOISON;
+
 	/*
 	 * For consistency in subsequent calls, make the faulted page always
 	 * locked.

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Add VM_FAULT_HWPOISON handling to the x86 page fault handler. This is very similar to VM_FAULT_OOM, the only difference is that a different si_code is passed to user space and the new addr_lsb field is initialized.

---
 arch/x86/mm/fault.c |   18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

Index: linux/arch/x86/mm/fault.c
===================================================================
@@ -189,6 +189,7 @@
 	info.si_errno	= 0;
 	info.si_code	= si_code;
 	info.si_addr	= (void __user *)address;
+	info.si_addr_lsb = si_code == BUS_MCEERR_AR ? PAGE_SHIFT : 0;
 
 	force_sig_info(si_signo, &info, tsk);
 }

@@ -827,10 +828,12 @@
 }
 
 static void
-do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address)
+do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
+	  unsigned int fault)
 {
 	struct task_struct *tsk = current;
 	struct mm_struct *mm = tsk->mm;
+	int code = BUS_ADRERR;
 
 	up_read(&mm->mmap_sem);
 

@@ -846,7 +849,14 @@
 	tsk->thread.error_code	= error_code;
 	tsk->thread.trap_no	= 14;
 
-	force_sig_info_fault(SIGBUS, BUS_ADRERR, address, tsk);
+#ifdef CONFIG_MEMORY_FAILURE
+	if (fault & VM_FAULT_HWPOISON) {
+		printk(KERN_ERR "MCE: Killing %s:%d due to hardware memory corruption\n",
+			tsk->comm, tsk->pid);
+		code = BUS_MCEERR_AR;
+	}
+#endif
+	force_sig_info_fault(SIGBUS, code, address, tsk);
 }
 
 static noinline void

@@ -856,8 +866,8 @@
 	if (fault & VM_FAULT_OOM) {
 		out_of_memory(regs, error_code, address);
 	} else {
-		if (fault & VM_FAULT_SIGBUS)
-			do_sigbus(regs, error_code, address);
+		if (fault & (VM_FAULT_SIGBUS|VM_FAULT_HWPOISON))
+			do_sigbus(regs, error_code, address, fault);
 		else
 			BUG();
 	}

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try_to_unmap currently has multiple modi (migration, munlock, normal unmap) which are selected by magic flag variables. The logic is not very straight forward, because each of these flag change multiple behaviours (e.g. migration turns off aging, not only sets up migration ptes etc.) Also the different flags interact in magic ways.

A later patch in this series adds another mode to try_to_unmap, so this becomes quickly unmanageable.

Replace the different flags with a action code (migration, munlock, munmap) and some additional flags as modifiers (ignore mlock, ignore aging). This makes the logic more straight forward and allows easier extension to new behaviours. Change all the caller to declare what they want to do.

This patch is supposed to be a nop in behaviour. If anyone can prove it is not that would be a bug.

---
 include/linux/rmap.h |   14 +++++++++++++-
 mm/migrate.c         |    2 +-
 mm/rmap.c            |   40 ++++++++++++++++++++++------------------
 mm/vmscan.c          |    2 +-
 4 files changed, 37 insertions(+), 21 deletions(-)

Index: linux/include/linux/rmap.h
===================================================================
@@ -84,7 +84,19 @@
  * Called from mm/vmscan.c to handle paging out
  */
 int page_referenced(struct page *, int is_locked, struct mem_cgroup *cnt);
-int try_to_unmap(struct page *, int ignore_refs);
+
+enum ttu_flags {
+	TTU_UNMAP = 0,			/* unmap mode */
+	TTU_MIGRATION = 1,		/* migration mode */
+	TTU_MUNLOCK = 2,		/* munlock mode */
+	TTU_ACTION_MASK = 0xff,
+
+	TTU_IGNORE_MLOCK = (1 << 8),	/* ignore mlock */
+	TTU_IGNORE_ACCESS = (1 << 9),	/* don't age */
+};
+#define TTU_ACTION(x) ((x) & TTU_ACTION_MASK)
+
+int try_to_unmap(struct page *, enum ttu_flags flags);
 
 /*
  * Called from mm/filemap_xip.c to unmap empty zero page

Index: linux/mm/rmap.c
===================================================================
@@ -755,7 +755,7 @@
  * repeatedly from either try_to_unmap_anon or try_to_unmap_file.
  */
 static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
-				int migration)
+				enum ttu_flags flags)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	unsigned long address;

@@ -777,11 +777,13 @@
 	 * If it's recently referenced (perhaps page_referenced
 	 * skipped over this mm) then we should reactivate it.
 	 */
-	if (!migration) {
+	if (!(flags & TTU_IGNORE_MLOCK)) {
 		if (vma->vm_flags & VM_LOCKED) {
 			ret = SWAP_MLOCK;
 			goto out_unmap;
 		}
+	}
+	if (!(flags & TTU_IGNORE_ACCESS)) {
 		if (ptep_clear_flush_young_notify(vma, address, pte)) {
 			ret = SWAP_FAIL;
 			goto out_unmap;

@@ -821,12 +823,12 @@
 			 * pte. do_swap_page() will wait until the migration
 			 * pte is removed and then restart fault handling.
 			 */
-			BUG_ON(!migration);
+			BUG_ON(TTU_ACTION(flags) != TTU_MIGRATION);
 			entry = make_migration_entry(page, pte_write(pteval));
 		}
 		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
 		BUG_ON(pte_file(*pte));
-	} else if (PAGE_MIGRATION && migration) {
+	} else if (PAGE_MIGRATION && (TTU_ACTION(flags) == TTU_MIGRATION)) {
 		/* Establish migration entry for a file page */
 		swp_entry_t entry;
 		entry = make_migration_entry(page, pte_write(pteval));

@@ -995,12 +997,13 @@
  * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
  * 'LOCKED.
  */
-static int try_to_unmap_anon(struct page *page, int unlock, int migration)
+static int try_to_unmap_anon(struct page *page, enum ttu_flags flags)
 {
 	struct anon_vma *anon_vma;
 	struct vm_area_struct *vma;
 	unsigned int mlocked = 0;
 	int ret = SWAP_AGAIN;
+	int unlock = TTU_ACTION(flags) == TTU_MUNLOCK;
 
 	if (MLOCK_PAGES && unlikely(unlock))
 		ret = SWAP_SUCCESS;	/* default for try_to_munlock() */

@@ -1016,7 +1019,7 @@
 				continue;  /* must visit all unlocked vmas */
 			ret = SWAP_MLOCK;  /* saw at least one mlocked vma */
 		} else {
-			ret = try_to_unmap_one(page, vma, migration);
+			ret = try_to_unmap_one(page, vma, flags);
 			if (ret == SWAP_FAIL || !page_mapped(page))
 				break;
 		}

@@ -1040,8 +1043,7 @@
 /**
  * try_to_unmap_file - unmap/unlock file page using the object-based rmap method
  * @page: the page to unmap/unlock
- * @unlock:  request for unlock rather than unmap [unlikely]
- * @migration:  unmapping for migration - ignored if @unlock
+ * @flags: action and flags
  *
  * Find all the mappings of a page using the mapping pointer and the vma chains
  * contained in the address_space struct it points to.

@@ -1053,7 +1055,7 @@
  * vm_flags for that VMA.  That should be OK, because that vma shouldn't be
  * 'LOCKED.
  */
-static int try_to_unmap_file(struct page *page, int unlock, int migration)
+static int try_to_unmap_file(struct page *page, enum ttu_flags flags)
 {
 	struct address_space *mapping = page->mapping;
 	pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);

@@ -1065,6 +1067,7 @@
 	unsigned long max_nl_size = 0;
 	unsigned int mapcount;
 	unsigned int mlocked = 0;
+	int unlock = TTU_ACTION(flags) == TTU_MUNLOCK;
 
 	if (MLOCK_PAGES && unlikely(unlock))
 		ret = SWAP_SUCCESS;	/* default for try_to_munlock() */

@@ -1077,7 +1080,7 @@
 				continue;	/* must visit all vmas */
 			ret = SWAP_MLOCK;
 		} else {
-			ret = try_to_unmap_one(page, vma, migration);
+			ret = try_to_unmap_one(page, vma, flags);
 			if (ret == SWAP_FAIL || !page_mapped(page))
 				goto out;
 		}

@@ -1102,7 +1105,8 @@
 			ret = SWAP_MLOCK;	/* leave mlocked == 0 */
 			goto out;		/* no need to look further */
 		}
-		if (!MLOCK_PAGES && !migration && (vma->vm_flags & VM_LOCKED))
+		if (!MLOCK_PAGES && !(flags & TTU_IGNORE_MLOCK) &&
+			(vma->vm_flags & VM_LOCKED))
 			continue;
 		cursor = (unsigned long) vma->vm_private_data;
 		if (cursor > max_nl_cursor)

@@ -1136,7 +1140,7 @@
 	do {
 		list_for_each_entry(vma, &mapping->i_mmap_nonlinear,
 						shared.vm_set.list) {
-			if (!MLOCK_PAGES && !migration &&
+			if (!MLOCK_PAGES && !(flags & TTU_IGNORE_MLOCK) &&
 			    (vma->vm_flags & VM_LOCKED))
 				continue;
 			cursor = (unsigned long) vma->vm_private_data;

@@ -1176,7 +1180,7 @@
 /**
  * try_to_unmap - try to remove all page table mappings to a page
  * @page: the page to get unmapped
- * @migration: migration flag
+ * @flags: action and flags
  *
  * Tries to remove all the page table entries which are mapping this
  * page, used in the pageout path.  Caller must hold the page lock.

@@ -1187,16 +1191,16 @@
  * SWAP_FAIL	- the page is unswappable
  * SWAP_MLOCK	- page is mlocked.
  */
-int try_to_unmap(struct page *page, int migration)
+int try_to_unmap(struct page *page, enum ttu_flags flags)
 {
 	int ret;
 
 	BUG_ON(!PageLocked(page));
 
 	if (PageAnon(page))
-		ret = try_to_unmap_anon(page, 0, migration);
+		ret = try_to_unmap_anon(page, flags);
 	else
-		ret = try_to_unmap_file(page, 0, migration);
+		ret = try_to_unmap_file(page, flags);
 	if (ret != SWAP_MLOCK && !page_mapped(page))
 		ret = SWAP_SUCCESS;
 	return ret;

@@ -1222,8 +1226,8 @@
 	VM_BUG_ON(!PageLocked(page) || PageLRU(page));
 
 	if (PageAnon(page))
-		return try_to_unmap_anon(page, 1, 0);
+		return try_to_unmap_anon(page, TTU_MUNLOCK);
 	else
-		return try_to_unmap_file(page, 1, 0);
+		return try_to_unmap_file(page, TTU_MUNLOCK);
 }
 #endif

Index: linux/mm/vmscan.c
===================================================================
@@ -666,7 +666,7 @@
 		 * processes. Try to unmap it here.
 		 */
 		if (page_mapped(page) && mapping) {
-			switch (try_to_unmap(page, 0)) {
+			switch (try_to_unmap(page, TTU_UNMAP)) {
 			case SWAP_FAIL:
 				goto activate_locked;
 			case SWAP_AGAIN:

Index: linux/mm/migrate.c
===================================================================
@@ -669,7 +669,7 @@
 	}
 
 	/* Establish migration ptes or remove ptes */
-	try_to_unmap(page, 1);
+	try_to_unmap(page, TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
 
 	if (!page_mapped(page))
 		rc = move_to_new_page(newpage, page);

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When a page has the poison bit set replace the PTE with a poison entry. This causes the right error handling to be done later when a process runs into it.

Also add a new flag to not do that (needed for the memory-failure handler later)

---
 include/linux/rmap.h |    1 +
 mm/rmap.c            |    9 ++++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

Index: linux/mm/rmap.c
===================================================================
@@ -801,7 +801,14 @@
 	/* Update high watermark before we lower rss */
 	update_hiwater_rss(mm);
 
-	if (PageAnon(page)) {
+	if (PageHWPoison(page) && !(flags & TTU_IGNORE_HWPOISON)) {
+		if (PageAnon(page))
+			dec_mm_counter(mm, anon_rss);
+		else if (!is_migration_entry(pte_to_swp_entry(*pte)))
+			dec_mm_counter(mm, file_rss);
+		set_pte_at(mm, address, pte,
+				swp_entry_to_pte(make_hwpoison_entry(page)));
+	} else if (PageAnon(page)) {
 		swp_entry_t entry = { .val = page_private(page) };
 
 		if (PageSwapCache(page)) {

Index: linux/include/linux/rmap.h
===================================================================
@@ -93,6 +93,7 @@
 
 	TTU_IGNORE_MLOCK = (1 << 8),	/* ignore mlock */
 	TTU_IGNORE_ACCESS = (1 << 9),	/* don't age */
+	TTU_IGNORE_HWPOISON = (1 << 10),/* corrupted page is recoverable */
 };
 #define TTU_ACTION(x) ((x) & TTU_ACTION_MASK)
 

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Bail out early in set_page_dirty for poisoned pages. We don't want any of the dirty accounting done or file system write back started, because the page will be just thrown away.

---
 mm/page-writeback.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux/mm/page-writeback.c
===================================================================
@@ -1277,6 +1277,10 @@
 {
 	struct address_space *mapping = page_mapping(page);
 
+	if (unlikely(PageHWPoison(page))) {
+		SetPageDirty(page);
+		return 0;
+	}
 	if (likely(mapping)) {
 		int (*spd)(struct page *) = mapping->a_ops->set_page_dirty;
 #ifdef CONFIG_BLOCK

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From: Wu Fengguang <[email protected]>

If memory corruption hits the free buddy pages, we can safely ignore them. No one will access them until page allocation time, then prep_new_page() will automatically check and isolate PG_hwpoison page for us (for 0-order allocation).

This patch expands prep_new_page() to check every component page in a high order page allocation, in order to completely stop PG_hwpoison pages from being recirculated.

Note that the common case -- only allocating a single page, doesn't do any more work than before. Allocating > order 0 does a bit more work, but that's relatively uncommon.

This simple implementation may drop some innocent neighbor pages, hopefully it is not a big problem because the event should be rare enough.

This patch adds some runtime costs to high order page users.

---
 mm/page_alloc.c |   22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

Index: linux/mm/page_alloc.c
===================================================================
@@ -633,12 +633,22 @@
  */
 static int prep_new_page(struct page *page, int order, gfp_t gfp_flags)
 {
-	if (unlikely(page_mapcount(page) |
-		(page->mapping != NULL)  |
-		(page_count(page) != 0)  |
-		(page->flags & PAGE_FLAGS_CHECK_AT_PREP))) {
-		bad_page(page);
-		return 1;
+	int i;
+
+	for (i = 0; i < (1 << order); i++) {
+		struct page *p = page + i;
+
+		if (unlikely(page_mapcount(p) |
+			(p->mapping != NULL)  |
+			(page_count(p) != 0)  |
+			(p->flags & PAGE_FLAGS_CHECK_AT_PREP))) {
+			/*
+			 * The whole array of pages will be dropped,
+			 * hopefully this is a rare and abnormal event.
+			 */
+			bad_page(p);
+			return 1;
+		}
 	}
 
 	set_page_private(page, 0);

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Add the high level memory handler that poisons pages
that got corrupted by hardware (typically by a two bit flip in a DIMM or a cache) on the Linux level. The goal is to prevent everyone from accessing these pages in the future.

This done at the VM level by marking a page hwpoisoned and doing the appropriate action based on the type of page it is.

The code that does this is portable and lives in mm/memory-failure.c

To quote the overview comment:

* High level machine check handler. Handles pages reported by the
* hardware as being corrupted usually due to a 2bit ECC memory or cache
* failure.
*
* This focuses on pages detected as corrupted in the background.
* When the current CPU tries to consume corruption the currently
* running process can just be killed directly instead. This implies
* that if the error cannot be handled for some reason it's safe to
* just ignore it because no corruption has been consumed yet. Instead
* when that happens another machine check will happen.
*
* Handles page cache pages in various states. The tricky part
* here is that we can access any page asynchronous to other VM
* users, because memory failures could happen anytime and anywhere,
* possibly violating some of their assumptions. This is why this code
* has to be extremely careful. Generally it tries to use normal locking
* rules, as in get the standard locks, even if that means the
* error handling takes potentially a long time.
*
* Some of the operations here are somewhat inefficient and have non
* linear algorithmic complexity, because the data structures have not
* been optimized for this case. This is in particular the case
* for the mapping from a vma to a process. Since this case is expected
* to be rare we hope we can get away with this.

There are in principle two strategies to kill processes on poison:
- just unmap the data and wait for an actual reference before killing
- kill as soon as corruption is detected.
Both have advantages and disadvantages and should be used in different situations. Right now both are implemented and can be switched with a new sysctl vm.memory_failure_early_kill The default is early kill.

The patch does some rmap data structure walking on its own to collect processes to kill. This is unusual because normally all rmap data structure knowledge is in rmap.c only. I put it here for now to keep everything together and rmap knowledge has been seeping out anyways

v2: Fix anon vma unlock crash (noticed by Johannes Weiner <[email protected]>) Handle pages on free list correctly (also noticed by Johannes)
Fix inverted try_to_release_page check (found by Chris Mason)
Add documentation for the new sysctl.
Various other cleanups/comment fixes.
v3: Use blockable signal for AO SIGBUS for better qemu handling. Numerous fixes from Fengguang Wu:
New code layout for the table (redone by AK)
Move the hwpoison bit setting before the lock (Fengguang Wu)
Some code cleanups (Fengguang Wu, AK)
Add missing lru_drain (Fengguang Wu)
Do more checks for valid mappings (inspired by patch from Fengguang) Handle free pages and fixes for clean pages (Fengguang)
Removed swap cache handling for now, needs more work
Better mapping checks to avoid races (Fengguang)
Fix swapcache (Fengguang)
Handle private2 pages too (Fengguang)
v4: Various fixes based on review comments from Nick Piggin Document locking order.
Improved comments.
Slightly improved description
Remove bogus hunk.
Wait properly for writeback pages (Nick Piggin)

---
 Documentation/sysctl/vm.txt |   21 +
 fs/proc/meminfo.c           |    9 
 include/linux/mm.h          |    4 
 kernel/sysctl.c             |   14 
 mm/Kconfig                  |    3 
 mm/Makefile                 |    1 
 mm/filemap.c                |    4 
 mm/memory-failure.c         |  720 ++++++++++++++++++++++++++++++++++++++++++++
 mm/rmap.c                   |    4 
 9 files changed, 778 insertions(+), 2 deletions(-)

Index: linux/mm/Makefile
===================================================================
@@ -38,3 +38,4 @@
 endif
 obj-$(CONFIG_QUICKLIST) += quicklist.o
 obj-$(CONFIG_CGROUP_MEM_RES_CTLR) += memcontrol.o page_cgroup.o
+obj-$(CONFIG_MEMORY_FAILURE) += memory-failure.o

Index: linux/include/linux/mm.h
===================================================================
@@ -1322,6 +1322,10 @@
 
 extern void *alloc_locked_buffer(size_t size);
 extern void free_locked_buffer(void *buffer, size_t size);
+
+extern void memory_failure(unsigned long pfn, int trapno);
+extern int sysctl_memory_failure_early_kill;
+extern atomic_long_t mce_bad_pages;
 extern void release_locked_buffer(void *buffer, size_t size);
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MM_H */

Index: linux/kernel/sysctl.c
===================================================================
@@ -1282,6 +1282,20 @@
 		.proc_handler	= &scan_unevictable_handler,
 	},
 #endif
+#ifdef CONFIG_MEMORY_FAILURE
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "memory_failure_early_kill",
+		.data		= &sysctl_memory_failure_early_kill,
+		.maxlen		= sizeof(vm_highmem_is_dirtyable),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec_minmax,
+		.strategy	= &sysctl_intvec,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
+#endif
+
 /*
  * NOTE: do not add new entries to this table unless you have read
  * Documentation/sysctl/ctl_unnumbered.txt

Index: linux/fs/proc/meminfo.c
===================================================================
@@ -97,7 +97,11 @@
 		"Committed_AS:   %8lu kB\n"
 		"VmallocTotal:   %8lu kB\n"
 		"VmallocUsed:    %8lu kB\n"
-		"VmallocChunk:   %8lu kB\n",
+		"VmallocChunk:   %8lu kB\n"
+#ifdef CONFIG_MEMORY_FAILURE
+		"BadPages:       %8lu kB\n"
+#endif
+		,
 		K(i.totalram),
 		K(i.freeram),
 		K(i.bufferram),

@@ -144,6 +148,9 @@
 		(unsigned long)VMALLOC_TOTAL >> 10,
 		vmi.used >> 10,
 		vmi.largest_chunk >> 10
+#ifdef CONFIG_MEMORY_FAILURE
+		,atomic_long_read(&mce_bad_pages) << (PAGE_SHIFT - 10)
+#endif
 		);
 
 	hugetlb_report_meminfo(m);

Index: linux/mm/Kconfig
===================================================================
@@ -226,6 +226,9 @@
 config MMU_NOTIFIER
 	bool
 
+config MEMORY_FAILURE
+	bool
+
 config NOMMU_INITIAL_TRIM_EXCESS
 	int "Turn on mmap() excess space trimming before booting"
 	depends on !MMU

Index: linux/Documentation/sysctl/vm.txt
===================================================================
@@ -32,6 +32,7 @@
 - legacy_va_layout
 - lowmem_reserve_ratio
 - max_map_count
+- memory_failure_early_kill
 - min_free_kbytes
 - min_slab_ratio
 - min_unmapped_ratio

@@ -53,7 +54,6 @@
 - vfs_cache_pressure
 - zone_reclaim_mode
 
-
 ==============================================================
 
 block_dump

@@ -275,6 +275,25 @@
 
 The default value is 65536.
 
+=============================================================
+
+memory_failure_early_kill:
+
+Control how to kill processes when uncorrected memory error (typically
+a 2bit error in a memory module) is detected in the background by hardware.
+
+1: Kill all processes that have the corrupted page mapped as soon as the
+corruption is detected.
+
+0: Only unmap the page from all processes and only kill a process
+who tries to access it.
+
+The kill is done using a catchable SIGBUS, so processes can handle this
+if they want to.
+
+This is only active on architectures/platforms with advanced machine
+check handling and depends on the hardware capabilities.
+
 ==============================================================
 
 min_free_kbytes:

Index: linux/mm/filemap.c
===================================================================
@@ -105,6 +105,10 @@
  *
  *  ->task->proc_lock
  *    ->dcache_lock		(proc_pid_lookup)
+ *
+ *  (code doesn't rely on that order, so you could switch it around)
+ *  ->tasklist_lock             (memory_failure, collect_procs_ao)
+ *    ->i_mmap_lock
  */
 
 /*

Index: linux/mm/rmap.c
===================================================================
@@ -36,6 +36,10 @@
  *                 mapping->tree_lock (widely used, in set_page_dirty,
  *                           in arch-dependent flush_dcache_mmap_lock,
  *                           within inode_lock in __sync_single_inode)
+ *
+ * (code doesn't rely on that order so it could be switched around)
+ * ->tasklist_lock
+ *   anon_vma->lock      (memory_failure, collect_procs_anon)
  */
 
 #include <linux/mm.h>

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Right, we can eliminate the PageWriteback() test when there is a wait_on_page_writeback().

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Normally the memory-failure.c code is enabled by the architecture, but for easier testing independent of architecture changes enable it unconditionally.

This should not be merged into mainline.

---
 mm/Kconfig |    2 ++
 1 file changed, 2 insertions(+)

Index: linux/mm/Kconfig
===================================================================
@@ -228,6 +228,8 @@
 
 config MEMORY_FAILURE
 	bool
+	default y
+	depends on MMU
 
 config NOMMU_INITIAL_TRIM_EXCESS
 	int "Turn on mmap() excess space trimming before booting"

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Impact: optional, useful for debugging

Add a new madvice sub command to inject poison for some pages in a process' address space. This is useful for testing the poison page handling.

Open issues:

- This patch allows root to tie up arbitary amounts of memory. Should this be disabled inside containers?
- There's a small race window between getting the page and injecting. The patch drops the ref count because otherwise memory_failure complains about dangling references. In theory with a multi threaded injector one could inject poison for a process foreign page this way. Not a serious issue right now.

v2: Use write flag for get_user_pages to make sure to always get a fresh page
v3: Don't request write mapping (Fengguang Wu)

---
 include/asm-generic/mman.h |    1 +
 mm/madvise.c               |   37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

Index: linux/mm/madvise.c
===================================================================
@@ -208,6 +208,38 @@
 	return error;
 }
 
+#ifdef CONFIG_MEMORY_FAILURE
+/*
+ * Error injection support for memory error handling.
+ */
+static int madvise_hwpoison(unsigned long start, unsigned long end)
+{
+	/*
+	 * RED-PEN
+	 * This allows to tie up arbitary amounts of memory.
+	 * Might be a good idea to disable it inside containers even for root.
+	 */
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	for (; start < end; start += PAGE_SIZE) {
+		struct page *p;
+		int ret = get_user_pages(current, current->mm, start, 1,
+						0, 0, &p, NULL);
+		if (ret != 1)
+			return ret;
+		put_page(p);
+		/*
+		 * RED-PEN page can be reused in a short window, but otherwise
+		 * we'll have to fight with the reference count.
+		 */
+		printk(KERN_INFO "Injecting memory failure for page %lx at %lx\n",
+		       page_to_pfn(p), start);
+		memory_failure(page_to_pfn(p), 0);
+	}
+	return 0;
+}
+#endif
+
 static long
 madvise_vma(struct vm_area_struct *vma, struct vm_area_struct **prev,
 		unsigned long start, unsigned long end, int behavior)

@@ -290,6 +322,11 @@
 	int write;
 	size_t len;
 
+#ifdef CONFIG_MEMORY_FAILURE
+	if (behavior == MADV_HWPOISON)
+		return madvise_hwpoison(start, start+len_in);
+#endif
+
 	write = madvise_need_mmap_write(behavior);
 	if (write)
 		down_write(&current->mm->mmap_sem);

Index: linux/include/asm-generic/mman.h
===================================================================
@@ -34,6 +34,7 @@
 #define MADV_REMOVE	9		/* remove these pages & resources */
 #define MADV_DONTFORK	10		/* don't inherit across fork */
 #define MADV_DOFORK	11		/* do inherit across fork */
+#define MADV_HWPOISON	12		/* hw poison the page (root only) */
 
 /* compatibility flags */
 #define MAP_FILE	0

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Useful for some testing scenarios, although specific testing is often done better through MADV_POISON

This can be done with the x86 level MCE injector too, but this interface allows it to do independently from low level x86 changes.

Open issues:

Should be disabled for cgroups.

---
 mm/Kconfig           |    4 ++++
 mm/Makefile          |    1 +
 mm/hwpoison-inject.c |   41 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+)

Index: linux/mm/hwpoison-inject.c
===================================================================
@@ -0,0 +1,41 @@
+/* Inject a hwpoison memory failure on a arbitary pfn */
+#include <linux/module.h>
+#include <linux/debugfs.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+
+static struct dentry *hwpoison_dir, *corrupt_pfn;
+
+static int hwpoison_inject(void *data, u64 val)
+{
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	printk(KERN_INFO "Injecting memory failure at pfn %Lx\n", val);
+	memory_failure(val, 18);
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(hwpoison_fops, NULL, hwpoison_inject, "%lli\n");
+
+static void pfn_inject_exit(void)
+{
+	if (hwpoison_dir)
+		debugfs_remove_recursive(hwpoison_dir);
+}
+
+static int pfn_inject_init(void)
+{
+	hwpoison_dir = debugfs_create_dir("hwpoison", NULL);
+	if (hwpoison_dir == NULL)
+		return -ENOMEM;
+	corrupt_pfn = debugfs_create_file("corrupt-pfn", 0600, hwpoison_dir,
+					  NULL, &hwpoison_fops);
+	if (corrupt_pfn == NULL) {
+		pfn_inject_exit();
+		return -ENOMEM;
+	}
+	return 0;
+}
+
+module_init(pfn_inject_init);
+module_exit(pfn_inject_exit);

Index: linux/mm/Kconfig
===================================================================
@@ -231,6 +231,10 @@
 	default y
 	depends on MMU
 
+config HWPOISON_INJECT
+	tristate "Poison pages injector"
+	depends on MEMORY_FAILURE && DEBUG_KERNEL
+
 config NOMMU_INITIAL_TRIM_EXCESS
 	int "Turn on mmap() excess space trimming before booting"
 	depends on !MMU

Index: linux/mm/Makefile
===================================================================
@@ -39,3 +39,4 @@
 obj-$(CONFIG_QUICKLIST) += quicklist.o
 obj-$(CONFIG_CGROUP_MEM_RES_CTLR) += memcontrol.o page_cgroup.o
 obj-$(CONFIG_MEMORY_FAILURE) += memory-failure.o
+obj-$(CONFIG_HWPOISON_INJECT) += hwpoison-inject.o

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Andrew please put it on the "Andi needs to justify his pageflags" non-path

I'm with Rik on this - we may have a few pageflags handy now but being slack with them for an obscure feature that can be done other ways and isn't performance critical is just lazy and bad planning for the long term.

Andi - "I'm doing it my way so nyahh, put it into .31" doesn't fly. If you want it in .31 convince Rik and me and others that its a good use of a pageflag.

Alan

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There's still plenty of space. Especially on 64bit it's an absolute non problem.

On 32bit the shortage of page flags was really
artificial because there were some caches put into ->flags, but these are largely obsolete to my understanding:
- discontigmem is gone (which cached the node)
- non vmap sparsemem is used a few times, but not on large systems where you have a lot of zones, so you are ok with only having a few bits for that
- if we really run out of bits on the sparsemem mapping it's easy enough to do another small hash table for this, similar to the discontig hash tables.

Also Christoph L. redid the dynamic allocation, so the boundaries are now dynamically growing/shrinking. This means that if an architecture doesn't use poison it doesn't use the bit.

Sorry, you guys also didn't do a very good job explaining why it is that big a problem to take a page flag. Yes I know it's popular folklore, but as far as I understand most of the reasons to be so stingy on them have disappeared over time anyways (but the folklore staid for some reason)

Anyways here's my pitch:

It's a straight forward concept expressable as a page flag. Lots of places need to check for it (we expect there will be more users in the future). Also even crash dumps should check for it, so it's important to have a clean interface.

Also it's an optional flag, if there's still an architecture around which needs special caches in ->flags then it's unlikely it will turn it on.

Also what's the alternative? Are you suggesting we should do huffman encoding on flags or something? That seemed just too ugly, especially to solve a non problem.

-Andi

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I thought a bit more about Alan's proposal of page flags compression for poisoned pages. I actually found more problems with it :-) (in addition to the points I wrote up in my earlier email on the topic)

Just wanted to write them up:

First some basics about hwpoison.

- HwPoisioning can come in at any time and at any state of the page.
- There can be multiple hwpoison events coming in for the same page in a short time window. This can happen for example when the hardware detects errors on different cache lines of a page, which can happen in some DIMM breakage scenarios.
The HwPoison bit serves as a synchronization point for this, it's essentially a lock for the hwpoison code (although no spinlock)
- HwPoison is high level code should only use portable primitives.

Alan proposed to use reserved|writeback to express hwpoisioning instead of an own bit.

- Now the first problem is that we don't have a portable primitive to set multiple bits atomically. cmpxchg() can be only used in architecture specific code. So it wouldn't be atomic in its locking function.

That means that all multiple bit variants are problematic, or at least would need a new global atomic primitive.

- Then you can actually have a page in writeback and poisoned. That is we can't stop writeback (we might at some point in the future), so the order the code works right now is:

set page poisoned
bail out if was already poisioned
do some other stuff
lock the page
wait for page writeback
(which just polls on the bit to clear)

Now the obvious problem is of course, if we used writeback|reserved, how would it it do the poison locking while the the page is still in writeback? The encoding would not be unique.

If we don't do that we would risk multiple memory_failures() on the same page, which has various issues.

So at least writeback|reserved doesn't work.

- Could we in theory find another weird bit combination that's truly impossible today ?
Probably, but it would be very hard to verify that this can truly never happen.

- Then I don't like it due to the fragility against other software bugs. Unless someone blasts 0xffs over the struct page (in which case treating it poisoned is probably a good thing anyways) then a separate bit is fairly robust against software bugs. Right now "impossible combinations" are used as a indication that something is wrong u with the page, to catch broken software.

If we gave meaning to previously impossible combinations then this robustness would be less. So a separate bit is generally more robust and doesn't take this away from the other code.

So using a separate bit is a sensible choice imho.

Hope this helps,

-Andi

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Could you make the feature 64-bit-only and use one of bits 32-63?

Did you consider making the poison tag external to the pageframe? Some hash(page*) into a bitmap or something? If suitably designed, such infrastructure could perhaps be reused to reclaim some existing page flags. Dave Hansen had such a patch a few years back. Or maybe it was Andy Whitcroft.

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We could, but these systems can run 32bit kernels too (although it's probably not a good idea). Ok it would be probably possible to make it 64bit only, but I would prefer to not do that.

Also even 32bit has still flags free and even if we run out there's an easy path to free more (see my earlier writeup)

So I don't see the pressing need to conserve every bit on 32bit.

I considered it at some point, but it would have complicated the code and I preferred to keep it simple. The poison handler should be relatively straight forward and do its work quickly otherwise it might not isolate the page before it's actually used.

-Andi

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hm. Maybe that should be proven sooner rather than later.

Well it's going to get complicated when we run out anyway. And run out we shall.

Plus we haven't looked into the complexity of the external flags yet.

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The SPARSEMEM code already has some fallback. I don't know if it works, but at least the code looks to be there.

* There are three possibilities for how page->flags get
* laid out. The first is for the normal case, without
* sparsemem. The second is for sparsemem when there is
* plenty of space for node and section. The last is when
* we have run out of space and have to fall back to an
* alternate (slower) way of determining the node. *

 * No sparsemem or sparsemem vmemmap: |       NODE     | ZONE | ... | FLAGS |
 * classic sparse with space for node:| SECTION | NODE | ZONE | ... | FLAGS |
 * classic sparse no space for node:  | SECTION |     ZONE    | ... | FLAGS |

/*
* If we did not store the node number in the page then we have to
* do a lookup in the section_to_node_table in order to find which
* node the page belongs to.
*/
#if MAX_NUMNODES <= 256
static u8 section_to_node_table[NR_MEM_SECTIONS] __cacheline_aligned; #else
static u16 section_to_node_table[NR_MEM_SECTIONS] __cacheline_aligned; #endif

The other part that could be added is to use a separate hash to go from page to SECTION (that would be very similar to the old discontig perfect hash I did to go from pfn to node), then the "SECTION" part would be free for reuse too.

Then you could use the full 32bits. On 32bit we're right now at 22, hwpoison would be 23. There's still some room.

It would be dumb to do external flags before you actually run out. After all what good are free bits?

-Andi

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