* + mm-memory-fix-truncation-removing-mapped-folios.patch added to mm-hotfixes-unstable branch
@ 2026-10-10 20:45 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2026-10-10 20:45 UTC (permalink / raw)
To: mm-commits, vbabka, surenb, rppt, qi.zheng, pfalcato, mhocko, ljs,
liam, jack, david, ayushr, baolin.wang, akpm
The patch titled
Subject: mm: memory: fix truncation removing mapped folios
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-memory-fix-truncation-removing-mapped-folios.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memory-fix-truncation-removing-mapped-folios.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days
------------------------------------------------------
From: Baolin Wang <baolin.wang@linux.alibaba.com>
Subject: mm: memory: fix truncation removing mapped folios
Date: Sat, 10 Oct 2026 15:22:18 +0800
Ayush reported an issue that shmem/tmpfs folios are still mapped when
truncating them on production:
"
BUG: Bad page cache in process ... pfn:...
page dumped because: still mapped when deleted
"
I reproduced the issue with Ayush's reproducer [1] with the latest mm-unstable
branch, and I got the following crash info. From the dump message, we can
see that truncate_inode_folio() is really trying to remove mapped folios,
which is incorrect.
"
[ 189.171977] page: refcount:3 mapcount:1 mapping:00000000fe168ee1
index:0x3880 pfn:0x191647
[ 189.171995] memcg:ffff0000cc073d40
[ 189.171997] aops:shmem_aops ino:3c01 dentry name(?):"memfd:runsc-memory"
[ 189.172006] flags: 0x17fffef0002022d(locked|referenced|uptodate|lru|
workingset|swapbacked|node=0|zone=2|lastcpupid=0x3ffff)
.......
[ 189.172017] page dumped because: VM_BUG_ON_FOLIO(folio_mapped(folio))
[ 189.172026] ------------[ cut here ]------------
[ 189.172027] kernel BUG at mm/filemap.c:155!
[ 189.172057] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
.......
[ 189.177721] CPU: 3 UID: 0 PID: 5521 Comm: repro Kdump: loaded Tainted:
7.3.0-rc4+ #277 PREEMPT(lazy)
.......
[ 189.183986] Call trace:
[ 189.184124] filemap_unaccount_folio+0xf0/0x1e8 (P)
[ 189.184391] __filemap_remove_folio+0x34/0x160
[ 189.184633] filemap_remove_folio+0x4c/0xb0
[ 189.184859] truncate_inode_folio+0x34/0x58
[ 189.185087] shmem_undo_range+0x220/0x658
[ 189.185308] shmem_fallocate+0x2f0/0x470
[ 189.185527] vfs_fallocate+0x128/0x328
......
[ 189.187078] el0t_64_sync+0x184/0x188
"
After analysis, I believe the race exists between truncation and
MADV_DONTNEED, and shmem's ->map_pages() merely makes the issue easier to
reproduce. Since MADV_DONTNEED synchronously releases the pagetable page
before calling tlb_flush_rmaps(), this could cause another thread's
truncation to skip zap_pte_range() (due to pmd is none) but still observe
the folio's mapcount as non-zero. A possible race scenario is as follows:
CPU 0 CPU 1
shmem_fallocate madvise_dontneed_single_vma
unmap_mapping_range .......
......
(filemap_map_pages() remap folios)
zap_pte_range
truncate_inode_folio zap_empty_pte_table(pmd clear)
unmap_mapping_folio
......
zap_pmd_range (saw pmd none, skip)
filemap_remove_folio
BUG_ON(folio_mapped)
tlb_flush_rmaps(remove rmaps)
To fix this issue, we should move the pmd clear operation (via
zap_empty_pte_table()) to after tlb_flush_rmaps() and add
smp_wmb()/smp_rmb() memory barriers, so that when we find a pmd_none()
while unmapping a folio without holding the PTL, the folio's mappings are
guaranteed to have been removed via tlb_flush_rmaps().
Link: https://lore.kernel.org/542c97482eae84f86704bcee56482633d1b32321.1791616645.git.baolin.wang@linux.alibaba.com
Link: https://lore.kernel.org/all/20260925065013.3682431-1-ayushr@modal.com/ [1]
Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Ayush Ranjan <ayushr@modal.com>
Closes: https://lore.kernel.org/all/20260924061708.1645968-1-ayushr@modal.com/
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Jan Kara <jack@suse.cz>
Cc: Pedro Falcato <pfalcato@suse.de>
---
mm/memory.c | 29 ++++++++++++++++++++---------
1 file changed, 20 insertions(+), 9 deletions(-)
--- a/mm/memory.c~mm-memory-fix-truncation-removing-mapped-folios
+++ a/mm/memory.c
@@ -2073,6 +2073,15 @@ retry:
}
} while (pte += nr, addr += PAGE_SIZE * nr, addr != end);
+ add_mm_rss_vec(mm, rss);
+ lazy_mmu_mode_disable();
+
+ /* Do the actual TLB flush before dropping ptl */
+ if (force_flush) {
+ tlb_flush_mmu_tlbonly(tlb);
+ tlb_flush_rmaps(tlb, vma);
+ }
+
/*
* Fast path: try to hold the pmd lock and unmap the PTE page.
*
@@ -2081,16 +2090,13 @@ retry:
* to ensure they are still none, thereby preventing the pte entries
* from being repopulated by another thread.
*/
- if (can_reclaim_pt && direct_reclaim && addr == end)
+ if (can_reclaim_pt && direct_reclaim && addr == end) {
+ /*
+ * The rmap changes need to be observed before PTEs get zapped.
+ * Pairs with smp_rmb() in zap_pmd_range().
+ */
+ smp_wmb();
direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval);
-
- add_mm_rss_vec(mm, rss);
- lazy_mmu_mode_disable();
-
- /* Do the actual TLB flush before dropping ptl */
- if (force_flush) {
- tlb_flush_mmu_tlbonly(tlb);
- tlb_flush_rmaps(tlb, vma);
}
pte_unmap_unlock(start_pte, ptl);
@@ -2145,6 +2151,11 @@ static inline unsigned long zap_pmd_rang
sync_with_folio_pmd_zap(tlb->mm, pmd);
}
if (pmd_none(*pmd)) {
+ /*
+ * Order this PMD read before the folio's mapcount read.
+ * Pairs with smp_wmb() in zap_pte_range().
+ */
+ smp_rmb();
addr = next;
continue;
}
_
Patches currently in -mm which might be from baolin.wang@linux.alibaba.com are
mm-memory-fix-truncation-removing-mapped-folios.patch
^ permalink raw reply [flat|nested] 2+ messages in thread
* + mm-memory-fix-truncation-removing-mapped-folios.patch added to mm-hotfixes-unstable branch
@ 2026-10-16 2:14 Andrew Morton
0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2026-10-16 2:14 UTC (permalink / raw)
To: mm-commits, vbabka, surenb, rppt, qi.zheng, pfalcato, mhocko, ljs,
liam, jack, david, ayushr, baolin.wang, akpm
The patch titled
Subject: mm: memory: fix truncation removing mapped folios
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-memory-fix-truncation-removing-mapped-folios.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memory-fix-truncation-removing-mapped-folios.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days
------------------------------------------------------
From: Baolin Wang <baolin.wang@linux.alibaba.com>
Subject: mm: memory: fix truncation removing mapped folios
Date: Fri, 16 Oct 2026 09:48:08 +0800
Ayush reported an issue that shmem/tmpfs folios are still mapped when
truncating them on production:
"
BUG: Bad page cache in process ... pfn:...
page dumped because: still mapped when deleted
"
I reproduced the issue with Ayush's reproducer [1] with the latest
mm-unstable branch, and I got the following crash info. From the dump
message, we can see that truncate_inode_folio() is really trying to remove
mapped folios, which is incorrect.
"
[ 189.171977] page: refcount:3 mapcount:1 mapping:00000000fe168ee1
index:0x3880 pfn:0x191647
[ 189.171995] memcg:ffff0000cc073d40
[ 189.171997] aops:shmem_aops ino:3c01 dentry name(?):"memfd:runsc-memory"
[ 189.172006] flags: 0x17fffef0002022d(locked|referenced|uptodate|lru|
workingset|swapbacked|node=0|zone=2|lastcpupid=0x3ffff)
.......
[ 189.172017] page dumped because: VM_BUG_ON_FOLIO(folio_mapped(folio))
[ 189.172026] ------------[ cut here ]------------
[ 189.172027] kernel BUG at mm/filemap.c:155!
[ 189.172057] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
.......
[ 189.177721] CPU: 3 UID: 0 PID: 5521 Comm: repro Kdump: loaded Tainted:
7.3.0-rc4+ #277 PREEMPT(lazy)
.......
[ 189.183986] Call trace:
[ 189.184124] filemap_unaccount_folio+0xf0/0x1e8 (P)
[ 189.184391] __filemap_remove_folio+0x34/0x160
[ 189.184633] filemap_remove_folio+0x4c/0xb0
[ 189.184859] truncate_inode_folio+0x34/0x58
[ 189.185087] shmem_undo_range+0x220/0x658
[ 189.185308] shmem_fallocate+0x2f0/0x470
[ 189.185527] vfs_fallocate+0x128/0x328
......
[ 189.187078] el0t_64_sync+0x184/0x188
"
After analysis, I believe the race exists between truncation and
MADV_DONTNEED, and shmem's ->map_pages() merely makes the issue easier to
reproduce. Since MADV_DONTNEED synchronously releases the pagetable page
before calling tlb_flush_rmaps(), this could cause another thread's
truncation to skip zap_pte_range() (due to pmd is none) but still observe
the folio's mapcount as non-zero. A possible race scenario is as follows:
CPU 0 CPU 1
shmem_fallocate madvise_dontneed_single_vma
unmap_mapping_range .......
......
(filemap_map_pages() remap folios)
zap_pte_range
truncate_inode_folio zap_empty_pte_table(pmd clear)
unmap_mapping_folio
......
zap_pmd_range (saw pmd none, skip)
filemap_remove_folio
BUG_ON(folio_mapped)
tlb_flush_rmaps(remove rmaps)
To fix this issue, we should move the pmd clear operation (via
zap_empty_pte_table()) to after tlb_flush_rmaps() and use
sync_with_folio_pmd_zap() to sync with concurrent zapping, so that when we
find a pmd_none() while unmapping a folio without holding the PTL, the
folio's mappings are guaranteed to have been removed via
tlb_flush_rmaps().
Link: https://lore.kernel.org/e2f76ed117b0c22a26b7688fd46a38c9d035336e.1792114445.git.baolin.wang@linux.alibaba.com
Link: https://lore.kernel.org/all/20260925065013.3682431-1-ayushr@modal.com/ [1]
Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Ayush Ranjan <ayushr@modal.com>
Closes: https://lore.kernel.org/all/20260924061708.1645968-1-ayushr@modal.com/
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Jan Kara <jack@suse.cz>
Cc: Pedro Falcato <pfalcato@suse.de>
---
mm/internal.h | 9 +++++----
mm/memory.c | 25 ++++++++++++-------------
2 files changed, 17 insertions(+), 17 deletions(-)
--- a/mm/internal.h~mm-memory-fix-truncation-removing-mapped-folios
+++ a/mm/internal.h
@@ -584,14 +584,15 @@ void free_pgtables(struct mmu_gather *tl
void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte);
/**
- * sync_with_folio_pmd_zap - sync with concurrent zapping of a folio PMD
+ * sync_with_folio_pmd_zap - sync with concurrent zapping of folios
* @mm: The mm_struct.
* @pmdp: Pointer to the pmd that was found to be pmd_none().
*
* When we find a pmd_none() while unmapping a folio without holding the PTL,
- * zap_huge_pmd() may have cleared the PMD but not yet modified the folio to
- * indicate that it's unmapped. Skipping the PMD without synchronization could
- * make folio unmapping code assume that unmapping failed.
+ * zap_huge_pmd() or zap_empty_pte_table() may have cleared the PMD but not
+ * yet modified the mapped folios to indicate they are unmapped. Skipping the
+ * PMD without synchronization could make folio unmapping code assume that
+ * unmapping failed.
*
* Wait for concurrent zapping to complete by grabbing the PTL.
*/
--- a/mm/memory.c~mm-memory-fix-truncation-removing-mapped-folios
+++ a/mm/memory.c
@@ -2073,6 +2073,15 @@ retry:
}
} while (pte += nr, addr += PAGE_SIZE * nr, addr != end);
+ add_mm_rss_vec(mm, rss);
+ lazy_mmu_mode_disable();
+
+ /* Do the actual TLB flush before dropping ptl */
+ if (force_flush) {
+ tlb_flush_mmu_tlbonly(tlb);
+ tlb_flush_rmaps(tlb, vma);
+ }
+
/*
* Fast path: try to hold the pmd lock and unmap the PTE page.
*
@@ -2083,15 +2092,6 @@ retry:
*/
if (can_reclaim_pt && direct_reclaim && addr == end)
direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval);
-
- add_mm_rss_vec(mm, rss);
- lazy_mmu_mode_disable();
-
- /* Do the actual TLB flush before dropping ptl */
- if (force_flush) {
- tlb_flush_mmu_tlbonly(tlb);
- tlb_flush_rmaps(tlb, vma);
- }
pte_unmap_unlock(start_pte, ptl);
/*
@@ -2139,12 +2139,11 @@ static inline unsigned long zap_pmd_rang
continue;
}
/* fall through */
- } else if (details && details->single_folio &&
- folio_test_pmd_mappable(details->single_folio) &&
- next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) {
- sync_with_folio_pmd_zap(tlb->mm, pmd);
}
+
if (pmd_none(*pmd)) {
+ if (details && details->single_folio)
+ sync_with_folio_pmd_zap(tlb->mm, pmd);
addr = next;
continue;
}
_
Patches currently in -mm which might be from baolin.wang@linux.alibaba.com are
mm-memory-fix-truncation-removing-mapped-folios.patch
^ permalink raw reply [flat|nested] 2+ messages in thread
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