linux-kernel - Re: [PATCHv2 4/5] mm/fault: Try to map the entire file folio in finish

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Message-ID: <hbse7gypxkaphzhdwkfw5nzu7oufcbd5iteyqw2nc7j4xptydh@rfympsitxagc>
Date: Fri, 19 Sep 2025 14:28:30 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, Hugh Dickins <hughd@...gle.com>, 
	Matthew Wilcox <willy@...radead.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	Michal Hocko <mhocko@...e.com>, Rik van Riel <riel@...riel.com>, 
	Harry Yoo <harry.yoo@...cle.com>, Johannes Weiner <hannes@...xchg.org>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Kiryl Shutsemau <kas@...nel.org>
Subject: Re: [PATCHv2 4/5] mm/fault: Try to map the entire file folio in
 finish_fault()

On Fri, Sep 19, 2025 at 01:40:35PM +0100, Kiryl Shutsemau wrote:
> From: Kiryl Shutsemau <kas@...nel.org>
> 
> The finish_fault() function uses per-page fault for file folios. This
> only occurs for file folios smaller than PMD_SIZE.
> 
> The comment suggests that this approach prevents RSS inflation.
> However, it only prevents RSS accounting. The folio is still mapped to
> the process, and the fact that it is mapped by a single PTE does not
> affect memory pressure. Additionally, the kernel's ability to map
> large folios as PMD if they are large enough does not support this
> argument.
> 
> When possible, map large folios in one shot. This reduces the number of
> minor page faults and allows for TLB coalescing.
> 
> Mapping large folios at once will allow the rmap code to mlock it on
> add, as it will recognize that it is fully mapped and mlocking is safe.
> 
> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>

Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>