Re: [PATCH V2 2/2] mm/khugepaged: fix race with folio split/free using temporary reference

[David Hildenbrand]

On 23.05.25 11:14, Shivank Garg wrote:
> hpage_collapse_scan_file() calls folio_expected_ref_count(), which in turn
> calls folio_mapcount(). folio_mapcount() checks folio_test_large() before
> proceeding to folio_large_mapcount(), but there is a race window where the
> folio may get split/freed between these checks, triggering:
>
>    VM_WARN_ON_FOLIO(!folio_test_large(folio), folio)
>
> Take a temporary reference to the folio in hpage_collapse_scan_file().
> This stabilizes the folio during refcount check and prevents incorrect
> large folio detection due to concurrent split/free.
>
> Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single value")
> Reported-by: syzbot+2b99589e33edbe9475ca@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@xxxxxxxxxx
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
> ---
> V1: https://lore.kernel.org/linux-mm/20250522093452.6379-1-shivankg@xxxxxxx
> ---

Assuming we have this as patch #1:

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb