[PATCH v3] mm: memory: fix truncation removing mapped folios

[PATCH v3] mm: memory: fix truncation removing mapped folios

[Baolin Wang]

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@modal.com/
Reported-by: Ayush Ranjan <ayushr@modal.com>
Closes: https://lore.kernel.org/all/20260924061708.1645968-1-ayushr@modal.com/
Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)")
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
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 |  9 +++++----
 mm/memory.c   | 25 ++++++++++++-------------
 2 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 0434dfcfc36f..6d8f494296a4 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -585,14 +585,15 @@ 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() 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.
  */
diff --git a/mm/memory.c b/mm/memory.c
index 6a8e7772b8d6..8f7ba427371b 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.
 	 *
@@ -2046,15 +2055,6 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 	 */
 	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 +2102,11 @@ 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;
 		}
-- 
2.47.3

Re: [PATCH v3] mm: memory: fix truncation removing mapped folios

[Qi Zheng]

On 10/16/26 9:48 AM, 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@modal.com/
> Reported-by: Ayush Ranjan <ayushr@modal.com>
> Closes: https://lore.kernel.org/all/20260924061708.1645968-1-ayushr@modal.com/
> Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)")
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> 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 |  9 +++++----
>   mm/memory.c   | 25 ++++++++++++-------------
>   2 files changed, 17 insertions(+), 17 deletions(-)

Thanks for the fix!

Acked-by: Qi Zheng <qi.zheng@linux.dev>