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

[Barry Song]

On Thu, Apr 10, 2025 at 9:05 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> 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()?

For instance, when a process, such as a game, just exits, it can be expected
that it won't be used again in the near future. As a result, demoting
its previously
unmapped file pages can improve performance.

Of course, for file folios mapped by multiple processes, such as
common .so files,
it's a different story. Typically, their mapcounts are always high.

>
> > 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