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FAILED: patch "[PATCH] mm/memory: do not populate page table entries beyond i_size" failed to apply to 6.12-stable tree

PATCH New [email protected] 2 participants 2 messages Nov 20, 2025

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#0 lore raw

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 74207de2ba10c2973334906822dc94d2e859ffc5
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025112023-nearest-surname-60bf@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 74207de2ba10c2973334906822dc94d2e859ffc5 Mon Sep 17 00:00:00 2001
From: Kiryl Shutsemau <[email protected]>
Date: Mon, 27 Oct 2025 11:56:35 +0000
Subject: [PATCH] mm/memory: do not populate page table entries beyond i_size

Patch series "Fix SIGBUS semantics with large folios", v3.

Accessing memory within a VMA, but beyond i_size rounded up to the next page size, is supposed to generate SIGBUS.

Darrick reported[1] an xfstests regression in v6.18-rc1. generic/749 failed due to missing SIGBUS. This was caused by my recent changes that try to fault in the whole folio where possible:

19773df031bc ("mm/fault: try to map the entire file folio in finish_fault()") 357b92761d94 ("mm/filemap: map entire large folio faultaround")

These changes did not consider i_size when setting up PTEs, leading to xfstest breakage.

However, the problem has been present in the kernel for a long time - since huge tmpfs was introduced in 2016. The kernel happily maps PMD-sized folios as PMD without checking i_size. And huge=always tmpfs allocates PMD-size folios on any writes.

I considered this corner case when I implemented a large tmpfs, and my conclusion was that no one in their right mind should rely on receiving a SIGBUS signal when accessing beyond i_size. I cannot imagine how it could be useful for the workload.

But apparently filesystem folks care a lot about preserving strict SIGBUS semantics.

Generic/749 was introduced last year with reference to POSIX, but no real workloads were mentioned. It also acknowledged the tmpfs deviation from the test case.

POSIX indeed says[3]:

References within the address range starting at pa and continuing for len bytes to whole pages following the end of an object shall result in delivery of a SIGBUS signal.

The patchset fixes the regression introduced by recent changes as well as more subtle SIGBUS breakage due to split failure on truncation.

This patch (of 2):

Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are supposed to generate SIGBUS.

Recent changes attempted to fault in full folio where possible. They did not respect i_size, which led to populating PTEs beyond i_size and breaking SIGBUS semantics.

Darrick reported generic/749 breakage because of this.

However, the problem existed before the recent changes. With huge=always tmpfs, any write to a file leads to PMD-size allocation. Following the fault-in of the folio will install PMD mapping regardless of i_size.

Fix filemap_map_pages() and finish_fault() to not install:
- PTEs beyond i_size;
- PMD mappings across i_size;

Make an exception for shmem/tmpfs that for long time intentionally mapped with PMDs across i_size.

Link https://lkml.kernel.org/r/[email protected] open here
Link https://lkml.kernel.org/r/[email protected] open here
Signed-off-by Kiryl Shutsemau <[email protected]>
Fixes 6795801366da ("xfs: Support large folios")
Reported-by "Darrick J. Wong" <[email protected]>
Cc Al Viro <[email protected]>
Cc Baolin Wang <[email protected]>
Cc Christian Brauner <[email protected]>
Cc Dave Chinner <[email protected]>
Cc David Hildenbrand <[email protected]>
Cc Hugh Dickins <[email protected]>
Cc Johannes Weiner <[email protected]>
Cc Liam Howlett <[email protected]>
Cc Lorenzo Stoakes <[email protected]>
Cc Matthew Wilcox (Oracle) <[email protected]>
Cc Michal Hocko <[email protected]>
Cc Mike Rapoport <[email protected]>
Cc Rik van Riel <[email protected]>
Cc Shakeel Butt <[email protected]>
Cc Suren Baghdasaryan <[email protected]>
Cc Vlastimil Babka <[email protected]>
Cc <[email protected]>
Signed-off-by Andrew Morton <[email protected]>
mm/filemap.c +20 -8
index 13f0259d993c..2f1e7e283a51 100644
@@ -3681,7 +3681,8 @@ static struct folio *next_uptodate_folio(struct xa_state *xas,
 static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
 			struct folio *folio, unsigned long start,
 			unsigned long addr, unsigned int nr_pages,
-			unsigned long *rss, unsigned short *mmap_miss)
+			unsigned long *rss, unsigned short *mmap_miss,
+			bool can_map_large)
 {
 	unsigned int ref_from_caller = 1;
 	vm_fault_t ret = 0;

@@ -3696,7 +3697,7 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
 	 * The folio must not cross VMA or page table boundary.
 	 */
 	addr0 = addr - start * PAGE_SIZE;
-	if (folio_within_vma(folio, vmf->vma) &&
+	if (can_map_large && folio_within_vma(folio, vmf->vma) &&
 	    (addr0 & PMD_MASK) == ((addr0 + folio_size(folio) - 1) & PMD_MASK)) {
 		vmf->pte -= start;
 		page -= start;

@@ -3811,13 +3812,27 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 	unsigned long rss = 0;
 	unsigned int nr_pages = 0, folio_type;
 	unsigned short mmap_miss = 0, mmap_miss_saved;
+	bool can_map_large;
 
 	rcu_read_lock();
 	folio = next_uptodate_folio(&xas, mapping, end_pgoff);
 	if (!folio)
 		goto out;
 
-	if (filemap_map_pmd(vmf, folio, start_pgoff)) {
+	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
+	end_pgoff = min(end_pgoff, file_end);
+
+	/*
+	 * Do not allow to map with PTEs beyond i_size and with PMD
+	 * across i_size to preserve SIGBUS semantics.
+	 *
+	 * Make an exception for shmem/tmpfs that for long time
+	 * intentionally mapped with PMDs across i_size.
+	 */
+	can_map_large = shmem_mapping(mapping) ||
+		file_end >= folio_next_index(folio);
+
+	if (can_map_large && filemap_map_pmd(vmf, folio, start_pgoff)) {
 		ret = VM_FAULT_NOPAGE;
 		goto out;
 	}

@@ -3830,10 +3845,6 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		goto out;
 	}
 
-	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
-	if (end_pgoff > file_end)
-		end_pgoff = file_end;
-
 	folio_type = mm_counter_file(folio);
 	do {
 		unsigned long end;

@@ -3850,7 +3861,8 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		else
 			ret |= filemap_map_folio_range(vmf, folio,
 					xas.xa_index - folio->index, addr,
-					nr_pages, &rss, &mmap_miss);
+					nr_pages, &rss, &mmap_miss,
+					can_map_large);
 
 		folio_unlock(folio);
 	} while ((folio = next_uptodate_folio(&xas, mapping, end_pgoff)) != NULL);
mm/memory.c +19 -1
index 74b45e258323..b59ae7ce42eb 100644
@@ -65,6 +65,7 @@
 #include <linux/gfp.h>
 #include <linux/migrate.h>
 #include <linux/string.h>
+#include <linux/shmem_fs.h>
 #include <linux/memory-tiers.h>
 #include <linux/debugfs.h>
 #include <linux/userfaultfd_k.h>

@@ -5501,8 +5502,25 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 			return ret;
 	}
 
+	if (!needs_fallback && vma->vm_file) {
+		struct address_space *mapping = vma->vm_file->f_mapping;
+		pgoff_t file_end;
+
+		file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
+
+		/*
+		 * Do not allow to map with PTEs beyond i_size and with PMD
+		 * across i_size to preserve SIGBUS semantics.
+		 *
+		 * Make an exception for shmem/tmpfs that for long time
+		 * intentionally mapped with PMDs across i_size.
+		 */
+		needs_fallback = !shmem_mapping(mapping) &&
+			file_end < folio_next_index(folio);
+	}
+
 	if (pmd_none(*vmf->pmd)) {
-		if (folio_test_pmd_mappable(folio)) {
+		if (!needs_fallback && folio_test_pmd_mappable(folio)) {
 			ret = do_set_pmd(vmf, folio, page);
 			if (ret != VM_FAULT_FALLBACK)
 				return ret;
#1 lore raw

PATCH 1/1 mm/memory: do not populate page table entries beyond i_size

Patch series "Fix SIGBUS semantics with large folios", v3.

Accessing memory within a VMA, but beyond i_size rounded up to the next page size, is supposed to generate SIGBUS.

Darrick reported[1] an xfstests regression in v6.18-rc1. generic/749 failed due to missing SIGBUS. This was caused by my recent changes that try to fault in the whole folio where possible:

19773df031bc ("mm/fault: try to map the entire file folio in finish_fault()") 357b92761d94 ("mm/filemap: map entire large folio faultaround")

These changes did not consider i_size when setting up PTEs, leading to xfstest breakage.

However, the problem has been present in the kernel for a long time - since huge tmpfs was introduced in 2016. The kernel happily maps PMD-sized folios as PMD without checking i_size. And huge=always tmpfs allocates PMD-size folios on any writes.

I considered this corner case when I implemented a large tmpfs, and my conclusion was that no one in their right mind should rely on receiving a SIGBUS signal when accessing beyond i_size. I cannot imagine how it could be useful for the workload.

But apparently filesystem folks care a lot about preserving strict SIGBUS semantics.

Generic/749 was introduced last year with reference to POSIX, but no real workloads were mentioned. It also acknowledged the tmpfs deviation from the test case.

POSIX indeed says[3]:

References within the address range starting at pa and continuing for len bytes to whole pages following the end of an object shall result in delivery of a SIGBUS signal.

The patchset fixes the regression introduced by recent changes as well as more subtle SIGBUS breakage due to split failure on truncation.

This patch (of 2):

Accesses within VMA, but beyond i_size rounded up to PAGE_SIZE are supposed to generate SIGBUS.

Recent changes attempted to fault in full folio where possible. They did not respect i_size, which led to populating PTEs beyond i_size and breaking SIGBUS semantics.

Darrick reported generic/749 breakage because of this.

However, the problem existed before the recent changes. With huge=always tmpfs, any write to a file leads to PMD-size allocation. Following the fault-in of the folio will install PMD mapping regardless of i_size.

Fix filemap_map_pages() and finish_fault() to not install:
- PTEs beyond i_size;
- PMD mappings across i_size;

Make an exception for shmem/tmpfs that for long time intentionally mapped with PMDs across i_size.

Link https://lkml.kernel.org/r/[email protected] open here
Link https://lkml.kernel.org/r/[email protected] open here
Signed-off-by Kiryl Shutsemau <[email protected]>
Fixes 6795801366da ("xfs: Support large folios")
Reported-by "Darrick J. Wong" <[email protected]>
Cc Al Viro <[email protected]>
Cc Baolin Wang <[email protected]>
Cc Christian Brauner <[email protected]>
Cc Dave Chinner <[email protected]>
Cc David Hildenbrand <[email protected]>
Cc Hugh Dickins <[email protected]>
Cc Johannes Weiner <[email protected]>
Cc Liam Howlett <[email protected]>
Cc Lorenzo Stoakes <[email protected]>
Cc Matthew Wilcox (Oracle) <[email protected]>
Cc Michal Hocko <[email protected]>
Cc Mike Rapoport <[email protected]>
Cc Rik van Riel <[email protected]>
Cc Shakeel Butt <[email protected]>
Cc Suren Baghdasaryan <[email protected]>
Cc Vlastimil Babka <[email protected]>
Cc <[email protected]>
Signed-off-by Andrew Morton <[email protected]>
(cherry picked from commit 74207de2ba10c2973334906822dc94d2e859ffc5)
Signed-off-by Kiryl Shutsemau <[email protected]>
---
 mm/filemap.c | 20 +++++++++++++++-----
 mm/memory.c  | 23 +++++++++++++++++++++--
 2 files changed, 36 insertions(+), 7 deletions(-)
mm/filemap.c +15 -5
index ec69fadf014c..d8d9c0f0beb6 100644
@@ -3653,13 +3653,27 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 	vm_fault_t ret = 0;
 	unsigned long rss = 0;
 	unsigned int nr_pages = 0, mmap_miss = 0, mmap_miss_saved, folio_type;
+	bool can_map_large;
 
 	rcu_read_lock();
 	folio = next_uptodate_folio(&xas, mapping, end_pgoff);
 	if (!folio)
 		goto out;
 
-	if (filemap_map_pmd(vmf, folio, start_pgoff)) {
+	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
+	end_pgoff = min(end_pgoff, file_end);
+
+	/*
+	 * Do not allow to map with PTEs beyond i_size and with PMD
+	 * across i_size to preserve SIGBUS semantics.
+	 *
+	 * Make an exception for shmem/tmpfs that for long time
+	 * intentionally mapped with PMDs across i_size.
+	 */
+	can_map_large = shmem_mapping(mapping) ||
+		file_end >= folio_next_index(folio);
+
+	if (can_map_large && filemap_map_pmd(vmf, folio, start_pgoff)) {
 		ret = VM_FAULT_NOPAGE;
 		goto out;
 	}

@@ -3672,10 +3686,6 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		goto out;
 	}
 
-	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
-	if (end_pgoff > file_end)
-		end_pgoff = file_end;
-
 	folio_type = mm_counter_file(folio);
 	do {
 		unsigned long end;
mm/memory.c +21 -2
index b6daa0e673a5..090e9c6f9992 100644
@@ -68,6 +68,7 @@
 #include <linux/gfp.h>
 #include <linux/migrate.h>
 #include <linux/string.h>
+#include <linux/shmem_fs.h>
 #include <linux/memory-tiers.h>
 #include <linux/debugfs.h>
 #include <linux/userfaultfd_k.h>

@@ -5088,6 +5089,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 	else
 		page = vmf->page;
 
+	folio = page_folio(page);
+
 	/*
 	 * check even for read faults because we might have lost our CoWed
 	 * page

@@ -5098,8 +5101,25 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 			return ret;
 	}
 
+	if (!needs_fallback && vma->vm_file) {
+		struct address_space *mapping = vma->vm_file->f_mapping;
+		pgoff_t file_end;
+
+		file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
+
+		/*
+		 * Do not allow to map with PTEs beyond i_size and with PMD
+		 * across i_size to preserve SIGBUS semantics.
+		 *
+		 * Make an exception for shmem/tmpfs that for long time
+		 * intentionally mapped with PMDs across i_size.
+		 */
+		needs_fallback = !shmem_mapping(mapping) &&
+			file_end < folio_next_index(folio);
+	}
+
 	if (pmd_none(*vmf->pmd)) {
-		if (PageTransCompound(page)) {
+		if (!needs_fallback && PageTransCompound(page)) {
 			ret = do_set_pmd(vmf, page);
 			if (ret != VM_FAULT_FALLBACK)
 				return ret;

@@ -5111,7 +5131,6 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
 			return VM_FAULT_OOM;
 	}
 
-	folio = page_folio(page);
 	nr_pages = folio_nr_pages(folio);
 
 	/*
Signature · 1 line
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