Re: [PATCH v4] mm: memory: fix truncation removing mapped folios

["David Hildenbrand (Arm)" <david@kernel.org>]

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@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)")
>>> Acked-by: Qi Zheng <qi.zheng@linux.dev>
>>> Reviewed-by: Pedro Falcato <pfalcato@suse.de>
>>> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>> 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@linux.alibaba.com/
> 
> 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