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

[Barry Song]

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>

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.

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