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

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Message-ID: <5815d9ed-e59f-4129-b20b-a0c07a2cf874@redhat.com>
Date: Sat, 24 May 2025 23:48:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Shivank Garg <shivankg@....com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, fengwei.yin@...el.com,
 bharata@....com, syzbot+2b99589e33edbe9475ca@...kaller.appspotmail.com
Subject: Re: [PATCH V2 2/2] mm/khugepaged: fix race with folio split/free
 using temporary reference

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@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Shivank Garg <shivankg@....com>
> ---
> V1: https://lore.kernel.org/linux-mm/20250522093452.6379-1-shivankg@amd.com
> ---

Assuming we have this as patch #1:

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb