Re: [PATCH V3 1/2] mm/khugepaged: fix race with folio split/free using temporary reference

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

On 2025/5/27 02:28, Shivank Garg wrote:
> hpage_collapse_scan_file() calls is_refcount_suitable(), 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. Use helper
> folio_expected_ref_count() + 1 to compare with folio_ref_count()
> instead of using is_refcount_suitable().
> 
> Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single value")
> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com
> Suggested-by: David Hildenbrand <david@redhat.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Shivank Garg <shivankg@amd.com>

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>