linux-kernel - Re: [RFC PATCH] mm: huge_memory: add folio_mark

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Message-ID: <282545E0-5B66-492D-B63F-838C6F066A22@nvidia.com>
Date: Tue, 08 Apr 2025 11:29:43 -0400
From: Zi Yan <ziy@...dia.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, david@...hat.com,
 21cnbao@...il.com, ryan.roberts@....com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC PATCH] mm: huge_memory: add folio_mark_accessed() when
 zapping file THP

On 8 Apr 2025, at 9:16, 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 more frequent refaults of PMD-sized file folios under memory
> pressure. Therefore, I am unsure whether the folio_mark_accessed() should be

How likely will the system get PMD-sized file folios when it is under
memory pressure? Johannes’ recent patch increases THP allocation successful
rate, maybe it was not happening before but will be after the patch?

> added for PMD-sized file folios?

Do you see any performance change after your patch?

>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  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);
>  		}
>
>  		spin_unlock(ptl);
> -- 
> 2.43.5


Best Regards,
Yan, Zi