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

[Dmitry Ilvokhin <d@ilvokhin.com>]

On Fri, Oct 03, 2025 at 03:41:05PM +0100, Kiryl Shutsemau wrote:
> On Fri, Oct 03, 2025 at 02:19:55PM +0000, Dmitry Ilvokhin wrote:
> > __mlock_folio() should update stats, when lruvec_add_folio() is called,
> 
> The update of stats is incidental to moving to unevicable LRU. But okay.
> 

Good point. I'll rephrase commit message in terms of unevicable
LRU instead of stat updates in v2.

> > but if folio_test_clear_lru() check failed, then __mlock_folio() gives
> > up early. From the other hand, folio_mark_accessed() calls
> > folio_activate() which also calls folio_test_clear_lru() down the line.
> > When folio_activate() successfully removed folio from LRU,
> > __mlock_folio() will not update any stats, which will lead to inaccurate
> > values in /proc/meminfo as well as cgroup memory.stat.
> > 
> > To prevent this case from happening 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.
> > 
> > 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
> 
> I think it is worth starting with the problem statement.
> 
> I like to follow this pattern of commit messages:
> 
> <Background, if needed>
> 
> <Issue statement>
> 
> <Proposed solution>
>

Thanks for suggestion, v2 commit message will much this pattern.

> > 
> > [1]: https://gist.github.com/ilvokhin/e50c3d2ff5d9f70dcbb378c6695386dd
> > 
> > Co-developed-by: Kiryl Shutsemau <kas@kernel.org>
> > Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> > Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>
> 
> Your Co-developed-by is missing. See submitting-patches.rst.
> 

I followed an example of a patch submitted by the From: author from
submitting-patches.rst. This example doesn't have Co-developed-by tag
from the From Author. That's being said, I found both cases usage in the
mm commit log, so I'll add mine Co-developed-by tag in the v2.

> > ---
> >  mm/swap.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 2260dcd2775e..f682f070160b 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -469,6 +469,16 @@ void folio_mark_accessed(struct folio *folio)
> >  		 * this list is never rotated or maintained, so marking an
> >  		 * unevictable page accessed has no effect.
> >  		 */
> > +	} else if (folio_test_mlocked(folio)) {
> > +		/*
> > +		 * Pages that are mlocked, but not yet on unevictable LRU.
> > +		 * They might be still in mlock_fbatch waiting to be processed
> > +		 * and activating it here might interfere with
> > +		 * mlock_folio_batch(). __mlock_folio() will fail
> > +		 * folio_test_clear_lru() check and give up. It happens because
> > +		 * __folio_batch_add_and_move() clears LRU flag, when adding
> > +		 * folio to activate batch.
> > +		 */
> >  	} else if (!folio_test_active(folio)) {
> >  		/*
> >  		 * If the folio is on the LRU, queue it for activation via
> > -- 
> > 2.47.3
> > 
> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov