Re: [PATCH v2] mm: memory: fix truncation removing mapped folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/12/26 6:23 PM, David Hildenbrand (Arm) wrote:
> On 10/10/26 09:22, 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)
> 
> That reminds me of sync_with_folio_pmd_zap() usage in page_vma_mapped_walk().
> 
> Isn't that the better approach?

I had considered this approach before, but it is not feasible. IIUC, the 
sync_with_folio_pmd_zap() is used to address the race with 
zap_huge_pmd(), and both the PMD clear and the folio mapcount decrement 
are performed under the PMD lock in zap_huge_pmd().

However, the race here is different: tlb_flush_rmaps() is under the PTE 
lock, while zap_empty_pte_table() is under the PMD lock, and reordering 
between the two functions is also possible. That is, the folio mapcount 
operation could execute after the acquire barrier (spin_lock()) of 
zap_empty_pte_table(), allowing a reader on CPU 0 to incorrectly observe 
that the PMD is none while mapcount > 0.

Also, we need to consider the possible reordering on the reader side on 
CPU 0, as discussed earlier [1].

[1] 
https://lore.kernel.org/all/1b77c2d7-acc6-476a-b5cd-547471a46409@linux.alibaba.com/