Re: [PATCH v4] mm: memory: fix truncation removing mapped folios
From: David Hildenbrand (Arm)
Date: Mon Oct 26 2026 - 07:57:45 EST
On 10/26/26 12:02, Baolin Wang wrote:
>
>
> On 10/26/26 6:00 PM, David Hildenbrand (Arm) wrote:
>> On 10/26/26 03:05, 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 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 | 28 +++++++++++++++-------------
>>> 2 files changed, 21 insertions(+), 17 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..6a93b7fa48b2 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 +2105,11 @@ 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;
>>>
>> Sorry if I sound annoyed, but why don't you keep the
>>
>> } else if (pmd_none(*pmd)) {
>>
>> above as I suggested?
>>
>> Because we fancy unnecessary PMD table lock cycles?
>
> Ah, sorry, I misunderstood what you meant before [1]. Yes, this is still correct
> and looks good to me.
>
> [1] https://lore.kernel.org/all/179c26ed-1764-4e5b-
> a7ee-047ddabbfea1@xxxxxxxxxxxxxxxxx/
>
> Do you want me to respin a new version or just apply the following diff?
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 6a93b7fa48b2..d9c54549b85b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2105,9 +2105,7 @@ static inline unsigned long zap_pmd_range(struct
> mmu_gather *tlb,
> continue;
> }
> /* fall through */
> - }
> -
> - if (pmd_none(*pmd)) {
> + } else if (pmd_none(*pmd)) {
> if (details && details->single_folio)
> sync_with_folio_pmd_zap(tlb->mm, pmd);
> addr = next;
That's not the correct fixup I think.
Please send a new version with the correct one ;)
--
Cheers,
David