linux-kernel - Re: [PATCH 1/2] mm/fault: Try to map the entire file folio in finish

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Message-ID: <6e6f596a-1817-45d6-b674-04e8aefde6d4@redhat.com>
Date: Thu, 18 Sep 2025 13:30:32 +0200
From: David Hildenbrand <david@...hat.com>
To: kirill@...temov.name, Andrew Morton <akpm@...ux-foundation.org>,
 Hugh Dickins <hughd@...gle.com>, Matthew Wilcox <willy@...radead.org>
Cc: 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>, Shakeel Butt <shakeel.butt@...ux.dev>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Kiryl Shutsemau <kas@...nel.org>, Baolin Wang <baolin.wang@...ux.alibaba.com>
Subject: Re: [PATCH 1/2] mm/fault: Try to map the entire file folio in
 finish_fault()

On 18.09.25 13:21, kirill@...temov.name 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>
> ---
>   mm/memory.c | 9 ++-------
>   1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 0ba4f6b71847..812a7d9f6531 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5386,13 +5386,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   
>   	nr_pages = folio_nr_pages(folio);
>   
> -	/*
> -	 * Using per-page fault to maintain the uffd semantics, and same
> -	 * approach also applies to non shmem/tmpfs faults to avoid
> -	 * inflating the RSS of the process.
> -	 */
> -	if (!vma_is_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
> -	    unlikely(needs_fallback)) {
> +	/* Using per-page fault to maintain the uffd semantics */
> +	if (unlikely(userfaultfd_armed(vma)) || unlikely(needs_fallback)) {
>   		nr_pages = 1;
>   	} else if (nr_pages > 1) {
>   		pgoff_t idx = folio_page_idx(folio, page);

I could have sworn that we recently discussed that.

Ah yes, there it is

https://lkml.kernel.org/r/a1c9ba0f-544d-4204-ad3b-60fe1be2ab32@linux.alibaba.com

CCing Baolin as he wanted to look into this.

-- 
Cheers

David / dhildenb