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

[Qi Zheng <qi.zheng@linux.dev>]

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>