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

[Baolin Wang]

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@xxxxxxxxxxxxxxxxx/