Re: [RFC PATCH v2 2/2] mm: improve large folio reuse for LRU-cached folios

[Barry Song]

On Wed, Jul 29, 2026 at 8:11 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 7/9/26 10:15, Barry Song (Xiaomi) wrote:
> > Large folios may now reside in the per-CPU LRU cache. Before
> > attempting to reuse them, drain the local LRU cache, which
> > can still be beneficial in cases where the folios are likely
> > to remain in this CPU's LRU cache:
> >
> > int main(int argc, char *argv[])
> > {
> >     int i;
> >     while (1) {
> >         volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE,
> >                                MAP_PRIVATE | MAP_ANONYMOUS,
> >                                -1, 0);
> >         for (int i = 0; i < SIZE / sizeof(int); i++)
> >             p[i] = i;
> >         madvise((void *)p, SIZE, MADV_PAGEOUT);
> >         if (!fork())
> >             _exit(0);
> >         for (int i = 0; i < SIZE / sizeof(int); i++)
> >             p[i] = i;
> >         munmap((void *)p, SIZE);
> >     }
> >     return 0;
> > }
> >
> > Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
> > ---
> >  mm/memory.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 5689b7cff76c..1d08ed5ba99b 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -4136,6 +4136,19 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
> >               folio_unlock(folio);
> >       }
> >
> > +     if (folio_may_be_lru_cached(folio) && !folio_test_lru(folio)) {
> > +             if (folio_ref_count(folio) > folio_large_mapcount(folio) + 1)
>
> I assume that can just be a "!=".

I was trying to guard against the case where a folio is not on
the LRU but also does not hold an extra reference. For example,
the folio is off the LRU, yet
folio_ref_count(folio) == folio_large_mapcount(folio).

In that case, we could simply reuse !folio_test_lru(folio).

Maybe such a case cannot actually happen, since paths like
isolate_folio() take an extra reference before removing the
folio from the LRU.

So we should be ok to just use folio_ref_count(folio) !=
folio_large_mapcount(folio) +1

Thanks
barry