linux-kernel - Re: [PATCH v3 3/3] khugepaged: Optimize collapse_pte_mapped

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Message-ID: <65476b79-e493-4f09-ab6b-3748ab679429@linux.alibaba.com>
Date: Wed, 23 Jul 2025 11:47:31 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org, david@...hat.com
Cc: ziy@...dia.com, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] khugepaged: Optimize collapse_pte_mapped_thp() by
 PTE batching



On 2025/7/22 23:05, Dev Jain wrote:
> Use PTE batching to optimize collapse_pte_mapped_thp().
> 
> On arm64, suppose khugepaged is scanning a pte-mapped 2MB THP for collapse.
> Then, calling ptep_clear() for every pte will cause a TLB flush for every
> contpte block. Instead, clear_ptes() does a contpte_try_unfold_partial()
> which will flush the TLB only for the (if any) starting and ending contpte
> block, if they partially overlap with the range khugepaged is looking at.
> 
> For all arches, there should be a benefit due to batching atomic operations
> on mapcounts due to folio_remove_rmap_ptes() and saving some calls.
> 
> Note that we do not need to make a change to the check
> "if (folio_page(folio, i) != page)"; if i'th page of the folio is equal
> to the first page of our batch, then i + 1, .... i + nr_batch_ptes - 1
> pages of the folio will be equal to the corresponding pages of our
> batch mapping consecutive pages.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>

With David's comments addressed, LGTM. Thanks.
Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>