Re: [PATCH v2] mm: skip folio_activate() for mlocked folios

[Nhat Pham]

On Mon, Oct 6, 2025 at 6:25 AM Dmitry Ilvokhin <d@xxxxxxxxxxxx> wrote:
>
> __mlock_folio() does not move folio to unevicable LRU, when
> folio_activate() removes folio from LRU.
>
> To prevent this case also check for folio_test_mlocked() in
> folio_mark_accessed(). If folio is not yet marked as unevictable, but
> already marked as mlocked, then skip folio_activate() call to allow
> __mlock_folio() to make all necessary updates. It should be safe to skip
> folio_activate() here, because mlocked folio should end up in
> unevictable LRU eventually anyway.
>
> To observe the problem mmap() and mlock() big file and check Unevictable
> and Mlocked values from /proc/meminfo. On freshly booted system without
> any other mlocked memory we expect them to match or be quite close.
>
> See below for more detailed reproduction steps. Source code of stat.c is
> available at [1].
>
>   $ head -c 8G < /dev/urandom > /tmp/random.bin
>
>   $ cc -pedantic -Wall -std=c99 stat.c -O3 -o /tmp/stat
>   $ /tmp/stat
>   Unevictable:     8389668 kB
>   Mlocked:         8389700 kB
>
>   Need to run binary twice. Problem does not reproduce on the first run,
>   but always reproduces on the second run.
>
>   $ /tmp/stat
>   Unevictable:     5374676 kB
>   Mlocked:         8389332 kB
>
> [1]: https://gist.github.com/ilvokhin/e50c3d2ff5d9f70dcbb378c6695386dd
>
> Co-developed-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> Signed-off-by: Dmitry Ilvokhin <d@xxxxxxxxxxxx>
> Acked-by: Usama Arif <usamaarif642@xxxxxxxxx>

Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>