linux-kernel - Re: [PATCHv3 4/6] mm/fault: Try to map the entire file folio in finish

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Message-ID: <b4093bcc-39c2-4cdc-a0d8-0c30ee8e1000@amd.com>
Date: Wed, 29 Oct 2025 10:22:46 +0530
From: "D, Suneeth" <Suneeth.D@....com>
To: Kiryl Shutsemau <kirill@...temov.name>, Andrew Morton
	<akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 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>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>, Kiryl Shutsemau
	<kas@...nel.org>
Subject: Re: [PATCHv3 4/6] mm/fault: Try to map the entire file folio in
 finish_fault()


Hi Kiryl Shutsemau,

On 9/23/2025 4:37 PM, 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.
> 

We run will-it-scale micro-benchmark as part of our weekly CI for Kernel 
Performance Regression testing between a stable vs rc kernel. We were 
able to observe drastic performance gain on AMD platforms (Turin and 
Bergamo) with running the will-it-scale-process-page-fault3 variant 
between the kernels v6.17 and v6.18-rc1 in the range of 322-400%. 
Bisecting further landed me onto this commit 
(19773df031bcc67d5caa06bf0ddbbff40174be7a) as the first commit to cause 
this gain.

The following were the machines' configuration and test parameters used:-

Model name:           AMD EPYC 128-Core Processor [Bergamo]
Thread(s) per core:   2
Core(s) per socket:   128
Socket(s):            1
Total online memory:  258G

Model name:           AMD EPYC 64-Core Processor [Turin]
Thread(s) per core:   2
Core(s) per socket:   64
Socket(s):            1
Total online memory:  258G

Test params:

     nr_task: [1 8 64 128 192 256]
     mode: process
     test: page_fault3
     kpi: per_process_ops
     cpufreq_governor: performance

The following are the stats after bisection:-

KPI       v6.17   %diff  v6.16-rc1  %diff  v6.17-with19773df031
-----     ------  -----  ---------  -----  --------------------
per_
process_  936152  +322   3954402    +339   4109353
ops

I have even checked the numbers built with the patch set[1] which was a 
fix to the regression reported[2], to see if the gain holds good and yes 
indeed it is.

                           per_process_ops   %diff (w.r.t baseline v6.17)
                           ---------------   ----------------------------
v6.17.0-withfixpatch:     3968637           +324

[1] 
http://lore.kernel.org/all/20251020163054.1063646-1-kirill@shutemov.name/
[2] https://lore.kernel.org/all/20251014175214.GW6188@frogsfrogsfrogs/

Recreation steps:

1) git clone https://github.com/antonblanchard/will-it-scale.git
2) git clone https://github.com/intel/lkp-tests.git
3) cd will-it-scale && git apply 
lkp-tests/programs/will-it-scale/pkg/will-it-scale.patch
4) make
5) python3 runtest.py page_fault3 25 process 0 0 1 8 64 128 192 256

NOTE: [5] is specific to machine's architecture. starting from 1 is the 
array of no.of tasks that you'd wish to run the testcase which here is 
no.cores per CCX, per NUMA node/ per Socket, nr_threads.

> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>   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);

Thanks and Regards,
Suneeth D