Re: [PATCH v5] mm: memory: fix truncation removing mapped folios
From: Baolin Wang
Date: Tue Oct 27 2026 - 06:37:21 EST
On 10/27/26 5:33 PM, David Hildenbrand (Arm) wrote:
On 10/27/26 10:03, Baolin Wang wrote:
On 10/27/26 5:00 PM, David Hildenbrand (Arm) wrote:
On 10/27/26 03:19, Baolin Wang wrote:
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().
[1] https://lore.kernel.org/all/20260925065013.3682431-1-ayushr@xxxxxxxxx/
Reported-by: Ayush Ranjan <ayushr@xxxxxxxxx>
Closes: https://lore.kernel.org/all/20260924061708.1645968-1-ayushr@xxxxxxxxx/
Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in
madvise(MADV_DONTNEED)")
Acked-by: Qi Zheng <qi.zheng@xxxxxxxxx>
Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
Changes from v4:
- Put the pmd_none() check in the 'else if' branch (per David).
Changes from v3:
- Collect review tags from Qi, Pedro and Lorenzo. Thanks.
- Update the related comments per Lorenzo.
Changes from v2:
- Use use sync_with_folio_pmd_zap() to sync with concurrent zapping (Per
David).
Changes from v1:
- Add an smp_rmb() to pair with the smp_wmb() (Per Lorenzo and Pedro).
---
mm/internal.h | 10 ++++++----
mm/memory.c | 30 +++++++++++++++---------------
2 files changed, 21 insertions(+), 19 deletions(-)
diff --git a/mm/internal.h b/mm/internal.h
index b1b0f29d2216..ffb8eaec90e3 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -584,14 +584,16 @@ void free_pgtables(struct mmu_gather *tlb, struct
unmap_desc *desc);
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() may have cleared the PMD but not yet updated the folio
+ * mapcount, and zap_empty_pte_table() clears the PMD only after the rmaps
+ * for the table have been removed, but nothing orders the pmd_none() read
+ * against later mapcount reads. Skipping the PMD without synchronization
+ * could make folio unmapping code assume that unmapping failed.
*
* Wait for concurrent zapping to complete by grabbing the PTL.
*/
diff --git a/mm/memory.c b/mm/memory.c
index e911e63ba1ed..d9c54549b85b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2036,6 +2036,15 @@ static unsigned long zap_pte_range(struct mmu_gather
*tlb,
}
} 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.
*
@@ -2043,18 +2052,12 @@ static unsigned long zap_pte_range(struct mmu_gather
*tlb,
* to hold the pmd lock failed, then we need to recheck all pte entries
* to ensure they are still none, thereby preventing the pte entries
* from being repopulated by another thread.
+ *
+ * Clear the PMD after the rmaps for the table have been removed, in case
+ * an incorrect mapcount is observed after unmapping the folio.
*/
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);
/*
@@ -2102,12 +2105,9 @@ static inline unsigned long zap_pmd_range(struct
mmu_gather *tlb,
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)) {
+ } else if (pmd_none(*pmd)) {
+ if (details && details->single_folio)
+ sync_with_folio_pmd_zap(tlb->mm, pmd);
addr = next;
continue;
}
} else if (pmd_none(*pmd) && details && details->single_folio)) {
sync_with_folio_pmd_zap(tlb->mm, pmd);
}
if (pmd_none(*pmd) {
addr = next;
continue;
}
Minimal change required, no?
No, keeping two pmd_none() checks cannot fix the race, as I replied before. [1]
Best to wait to submit new versions if there are open discussions? Because now
we have the same discussion in two places.
Sure.
I think you're right about the race, the issue is that we check for
pmd_is_huge(), not pmd_present() which confused me.
Clearest to me would be to not drop the second pmd_none() check but move it
where it belongs: if splitting zapped the PMD.
@@ -2101,13 +2104,15 @@ static inline unsigned long zap_pmd_range(struct
mmu_gather *tlb,
addr = next;
continue;
}
+ /* We already synchronized with concurrent zapping. */
+ if (pmd_none(*pmd)) {
+ addr = next;
+ 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)) {
+ } else if (pmd_none(*pmd)) {
+ if (details && details->single_folio)
+ sync_with_folio_pmd_zap(tlb->mm, pmd);
addr = next;
continue;
}
IMHO, I'm not a big fan of adding two pmd_none() checks just to avoid an uncommon concurrent sync. If you don't like the approach in v4 [1] either, I'll follow your preferred coding style.
@@ -2102,12 +2105,11 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
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;
}
[1] https://lore.kernel.org/all/4cc521a118564b4145edf0324066ab532fb41181.1792979870.git.baolin.wang@xxxxxxxxxxxxxxxxx/#t
Removing the "if (pmd_none(*pmd)) {" in your variant is what confused me,
because it looks like we no longer happen the case where we zapped the PMD
ourselves.
Because zap_pte_range() will return the same addr and check the none PMD again in zap_pmd_range() in this case, I thought you wanted this simple version, considering keeping two pmd_none() checks is incorrect.