Re: [PATCH] mm: huge_memory: add folio_mark_accessed() when zapping file THP

[David Hildenbrand]

On 09.04.25 11:49, Baolin Wang wrote:
> On 2025/4/9 17:46, David Hildenbrand wrote:
> > On 09.04.25 11:38, Baolin Wang wrote:
> > > When investigating performance issues during file folio unmap, I
> > > noticed some
> > > behavioral differences in handling non-PMD-sized folios and PMD-sized
> > > folios.
> > > For non-PMD-sized file folios, it will call folio_mark_accessed() to
> > > mark the
> > > folio as having seen activity, but this is not done for PMD-sized folios.
> > >
> > > This might not cause obvious issues, but a potential problem could be
> > > that,
> > > it might lead to reclaim hot file folios under memory pressure, as quoted
> > > from Johannes:
> > >
> > > "
> > > Sometimes file contents are only accessed through relatively short-lived
> > > mappings. But they can nevertheless be accessed a lot and be hot. It's
> > > important to not lose that information on unmap, and end up kicking out a
> > > frequently used cache page.
> > > "
> > >
> > > Therefore, we should also add folio_mark_accessed() for PMD-sized file
> > > folios when unmapping.
> > >
> > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > Acked-by: Zi Yan <ziy@xxxxxxxxxx>
> > > ---
> > > Changes from RFC:
> > >    - Update the commit message, per Johannes.
> > >    - Collect Acked tags from Johannes and Zi. Thanks.
> > > ---
> > >    mm/huge_memory.c | 4 ++++
> > >    1 file changed, 4 insertions(+)
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 2a47682d1ab7..955781b4e946 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -2260,6 +2260,10 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct
> > > vm_area_struct *vma,
> > >                    zap_deposited_table(tlb->mm, pmd);
> > >                add_mm_counter(tlb->mm, mm_counter_file(folio),
> > >                           -HPAGE_PMD_NR);
> > > +
> > > +            if (flush_needed && pmd_young(orig_pmd) &&
> > > +                likely(vma_has_recency(vma)))
> > > +                folio_mark_accessed(folio);
> >
> > So the flush_needed check is really just a pmd_present() check. (the
> > latter would be clearer, but I don't mind)
>
> Yes, we've already checked pmd_present() before, so I assume the
> flush_needed check is cheaper:)

Maybe :)

--
Cheers,

David / dhildenb