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

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Message-ID: <39176ab9-33d5-4919-b368-2fd2467f08fe@redhat.com>
Date: Wed, 9 Apr 2025 11:46:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org
Cc: willy@...radead.org, hannes@...xchg.org, 21cnbao@...il.com,
 ryan.roberts@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: huge_memory: add folio_mark_accessed() when zapping
 file THP

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@...ux.alibaba.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Zi Yan <ziy@...dia.com>
> ---
> 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)

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb