Re: [RFC PATCH 1/3] mm: avoid unnecessary lru drain for wp_can_reuse_anon_folio()

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Jun 11, 2026 at 11:09:43AM -0700, Shakeel Butt wrote:
> On Thu, Jun 11, 2026 at 06:51:22PM +0800, 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.
> > 
> > Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> > ---
> >  mm/memory.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 56be920c56d7..487a34377a7b 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;
> 
> In your experiments, how much amount of drains were reduced due to this specific
> check?
> 
> I wonder if that data can motivate to introduce lru_add_drain_folio(folio) which
> only drains if the given folio is in the local lru_add cache.

Actually if we can peek into lru_add cache and folio_ref_count(folio) is exactly
equal to (2 + folio_test_swapcache(folio)) and folio is not on LRU then we can
just reuse folio if it is in lru_add cache without draining, right?

> 
> >  		/*
> >  		 * We cannot easily detect+handle references from
> >  		 * remote LRU caches or references to LRU folios.
> >  		 */
> >  		lru_add_drain();
> > +	}
> >  	if (folio_ref_count(folio) > 1 + folio_test_swapcache(folio))
> >  		return false;
> >  	if (!folio_trylock(folio))
> > -- 
> > 2.39.3 (Apple Git-146)
> >