Re: [PATCH 1/1] mm: avoid processing mlocked THPs in deferred split shrinker

[Lance Yang]

On 2025/9/8 15:38, David Hildenbrand wrote:
> On 08.09.25 06:07, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
>
> Subject should likely be more specific:
>
> mm: skip mlocked THPs that are underused early in deferred_split_scan()

Right, that's a much better and more precise subject. Thanks!

> > When a new THP is faulted in or collapsed, it is unconditionally added to
> > the deferred split queue. If this THP is subsequently mlocked, it remains
> > on the queue but is removed from the LRU and marked unevictable.
> >
> > During memory reclaim, deferred_split_scan() will still pick up this 
> > large
> > folio. Because it's not partially mapped, it will proceed to call
> > thp_underused() and then attempt to split_folio() to free all zero-filled
> > subpages.
> >
> > This is a pointless waste of CPU cycles. The folio is mlocked and
> > unevictable, so any attempt to reclaim memory from it via splitting is
> > doomed to fail.
>
> I think the whole description is a bit misleading: we're not reclaiming
> memory from fully-mapped THPs even when they are underused, because it
> could violate mlock() semantics where we don't want a page fault+memory 
> allocation on next access.
>
> So something like the following might be clearer.
>
> "When we stumble over a fully-mapped THP in the deferred shrinker, it 
> does not make sense trying to detect whether it is underused, because 
> try_to_map_unused_to_zeropage(), called while splitting the folio, will 
> not actually replace any zero-ed pages by the shared zeropage.
>
> Splitting the folio in that case does not make any sense, so let's not 
> even scan if the folio is underused.
> "

Nice, that makes it much clearer. My understanding was indeed imprecise.

> If I run my reproducer from [1] and mlock() the pages just after 
> allocating them, then I essentially get
>
> AnonHugePages:   1048576 kB
>
> converted to
>
> Anonymous:       1048580 kB
>
> Which makes sense (no memory optimized out) as discussed above.

Yes, my reproducer also shows exactly that. It's clear a lot of work is
done but no memory is actually optimized out ;)

> [1] https://lkml.kernel.org/r/20250905141137.3529867-1-david@xxxxxxxxxx
>
> > So, let's add an early folio_test_mlocked() check to skip this case.
> >
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
> >   mm/huge_memory.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 77f0c3417973..d2e84015d6b4 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -4183,6 +4183,9 @@ static unsigned long deferred_split_scan(struct 
> > shrinker *shrink,
> >           bool underused = false;
> >           if (!folio_test_partially_mapped(folio)) {
> > +            /* An mlocked folio is not a candidate for the shrinker. */
>
> /*
>   * See try_to_map_unused_to_zeropage(): we cannot optimize zero-filled
>   * pages after splitting an mlocked folio.
>   */

Got it. I'll update the changelog and this comment as suggested.

> > +            if (folio_test_mlocked(folio))
> > +                goto next;
> >               underused = thp_underused(folio);
> >               if (!underused)
> >                   goto next;

Cheers,
Lance