linux-kernel - Re: [PATCH v6 5/6] mm: vmscan: Avoid split during shrink_folio

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Message-ID: <6653ce52-53a2-4d91-b98f-50f1608ae59c@redhat.com>
Date: Fri, 5 Apr 2024 12:42:35 +0200
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>, Lance Yang <ioworker0@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v6 5/6] mm: vmscan: Avoid split during shrink_folio_list()

On 03.04.24 13:40, 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.
> 
> THP_SWPOUT and THP_SWPOUT_FALLBACK counters should continue to count
> events only for PMD-mappable folios to avoid user confusion. THP_SWPOUT
> already has the appropriate guard. Add a guard for THP_SWPOUT_FALLBACK.
> It may be appropriate to add per-size counters in future.
> 
> Reviewed-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Barry Song <v-songbaohua@...o.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>   mm/vmscan.c | 17 +++++++++++------
>   1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 00adaf1cb2c3..ffc4553c8615 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1223,11 +1223,12 @@ 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 right
> +					 * away. We can free the unmapped pages
> +					 * without IO.
>   					 */
> -					if (!folio_entire_mapcount(folio) &&
> +					if (data_race(!list_empty(
> +						&folio->_deferred_list)) &&

Please violate the 80 chars rule where reasonable.

"
Statements longer than 80 columns should be broken into sensible chunks, 
unless exceeding 80 columns significantly increases readability and does 
not hide information.
"


Likely some of that code here should be refactored into readable 
sub-functions.

codingstyle also mentions

"The answer to that is that if you need more than 3 levels of 
indentation, you're screwed anyway, and should fix your program."

-- 
Cheers,

David / dhildenb