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

[Baolin Wang]

On 2025/4/10 16:14, Barry Song wrote:
> On Wed, Apr 9, 2025 at 1:16 AM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> 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 more frequent refaults of PMD-sized file folios under memory
> > pressure. Therefore, I am unsure whether the folio_mark_accessed() should be
> > added for PMD-sized file folios?
> >
> > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/huge_memory.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 6ac6d468af0d..b3ade7ac5bbf 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2262,6 +2262,10 @@ 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);
>
> Acked-by: Barry Song <baohua@xxxxxxxxxx>

Thanks.

> I also came across an interesting observation: on a memory-limited system,
> demoting unmapped file folios in the LRU—specifically when their mapcount
> drops from 1 to 0—can actually improve performance.

These file folios are used only once? Can folio_set_dropbehind() be used 
to optimize it, which can avoid the LRU activity movement in 
folio_mark_accessed()?

> If others have observed the same behavior, we might not need to mark them
> as accessed in that scenario.
>
> >                  }
> >
> >                  spin_unlock(ptl);
> > --
> > 2.43.5
>
> Thanks
> barry