linux-kernel - Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio

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Date: Fri, 15 Mar 2024 11:43:19 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Huang Ying <ying.huang@...el.com>,
 Gao Xiang <xiang@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
 Yang Shi <shy828301@...il.com>, Michal Hocko <mhocko@...e.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Barry Song <21cnbao@...il.com>,
 Chris Li <chrisl@...nel.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

On 11.03.24 16:00, Ryan Roberts wrote:
> Now that swap supports storing all mTHP sizes, avoid splitting large
> folios before swap-out. This benefits performance of the swap-out path
> by eliding split_folio_to_list(), which is expensive, and also sets us
> up for swapping in large folios in a future series.
> 
> If the folio is partially mapped, we continue to split it since we want
> to avoid the extra IO overhead and storage of writing out pages
> uneccessarily.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>   mm/vmscan.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index cf7d4cf47f1a..0ebec99e04c6 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>   					if (!can_split_folio(folio, NULL))
>   						goto activate_locked;
>   					/*
> -					 * Split folios without a PMD map right
> -					 * away. Chances are some or all of the
> -					 * tail pages can be freed without IO.
> +					 * Split partially mapped folios map
> +					 * right away. Chances are some or all
> +					 * of the tail pages can be freed
> +					 * without IO.
>   					 */
> -					if (!folio_entire_mapcount(folio) &&
> +					if (!list_empty(&folio->_deferred_list) &&
>   					    split_folio_to_list(folio,
>   								folio_list))
>   						goto activate_locked;

Not sure if we might have to annotate that with data_race().

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

-- 
Cheers,

David / dhildenb