Re: [PATCH v2 1/4] mm: avoid unnecessary lru drain for wp_can_reuse_anon_folio()

[Barry Song]

On Wed, Jun 24, 2026 at 11:02 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 6/24/26 01:16, Barry Song (Xiaomi) wrote:
> > We always unconditionally drain the LRU before retrying anon folio
> > reuse in wp_can_reuse_anon_folio(). Instead, assume !LRU anon folios
> > are in lru_cache, and use the refcount to avoid many unnecessary LRU
> > drains.
> >
> > Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> > Reviewed-by: Baoquan He <baoquan.he@xxxxxxxxx>
> > Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> > ---
> >  mm/memory.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index ff338c2abe92..f6848f4234a6 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4193,12 +4193,18 @@ static bool wp_can_reuse_anon_folio(struct folio *folio,
> >        */
> >       if (folio_test_ksm(folio) || folio_ref_count(folio) > 3)
> >               return false;
> > -     if (!folio_test_lru(folio))
> > +     if (!folio_test_lru(folio)) {
> > +             /*
> > +              * Assume folio is on lru_cache and holds a cache reference.
> > +              */
> > +             if (folio_ref_count(folio) > 2 + folio_test_swapcache(folio))
> > +                     return false;
>
> I'm not keen on making this function even uglier, so no, not like that.
>
> We have the earlier "folio_ref_count(folio) > 3" check.
>
> In which scenarios can you trigger this such that we would care?
>
> If the answer is "I don't know" there is no reason for a change.

As I replied to Shakeel in the v1 discussion [1], this can avoid a
large number of drains, both during Ubuntu boot and under normal
workloads:

"I booted the system into Ubuntu, and after

boot completed I observed:

wp_reuse_skipped_drain: 5542
do_swap_skipped_drain: 0

Then I built the kernel in a 1GB memcg using zRAM swap, and observed:

wp_reuse_skipped_drain: 25017
do_swap_skipped_drain: 43595

So in summary, even without swap-in we can save a significant number of
drains in wp_can_reuse_anon_folio(). With heavy swap-in workloads, most
of the savings come from avoiding drains in do_swap_page(), while we
still see a substantial number of skipped drains in wp_can_reuse_anon_folio()."

This is exactly the case where folio_ref_count(folio) == 3 and
!folio_test_swapcache(folio). That is why checking
folio_ref_count(folio) > 3 does not help.

[1] https://lore.kernel.org/linux-mm/CAGsJ_4yCeAmpfo_8ef4MrRyRD_UtV-xGm6F4GpSjvLVPQsbiEg@xxxxxxxxxxxxxx/

Best Regards
Barry