linux-kernel - Re: [PATCH v2] gup: optimize longterm pin_user

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Message-ID: <5023a912-1ee9-4082-8656-56e004623367@redhat.com>
Date: Wed, 4 Jun 2025 10:12:00 +0200
From: David Hildenbrand <david@...hat.com>
To: lizhe.67@...edance.com, akpm@...ux-foundation.org, jgg@...pe.ca,
 jhubbard@...dia.com, peterx@...hat.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, dev.jain@....com,
 muchun.song@...ux.dev
Subject: Re: [PATCH v2] gup: optimize longterm pin_user_pages() for large
 folio

On 04.06.25 05:15, lizhe.67@...edance.com wrote:
> From: Li Zhe <lizhe.67@...edance.com>
> 
> In the current implementation of the longterm pin_user_pages() function,
> we invoke the collect_longterm_unpinnable_folios() function. This function
> iterates through the list to check whether each folio belongs to the
> "longterm_unpinnabled" category. The folios in this list essentially
> correspond to a contiguous region of user-space addresses, with each folio
> representing a physical address in increments of PAGESIZE. If this
> user-space address range is mapped with large folio, we can optimize the
> performance of function pin_user_pages() by reducing the frequency of
> memory accesses using READ_ONCE. This patch leverages this approach to
> achieve performance improvements.
> 
> The performance test results obtained through the gup_test tool from the
> kernel source tree are as follows. We achieve an improvement of over 70%
> for large folio with pagesize=2M. For normal page, we have only observed
> a very slight degradation in performance.
> 
> Without this patch:
> 
>      [root@...alhost ~] ./gup_test -HL -m 8192 -n 512
>      TAP version 13
>      1..1
>      # PIN_LONGTERM_BENCHMARK: Time: get:13623 put:10799 us#
>      ok 1 ioctl status 0
>      # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
>      [root@...alhost ~]# ./gup_test -LT -m 8192 -n 512
>      TAP version 13
>      1..1
>      # PIN_LONGTERM_BENCHMARK: Time: get:129733 put:31753 us#
>      ok 1 ioctl status 0
>      # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
> With this patch:
> 
>      [root@...alhost ~] ./gup_test -HL -m 8192 -n 512
>      TAP version 13
>      1..1
>      # PIN_LONGTERM_BENCHMARK: Time: get:4075 put:10792 us#
>      ok 1 ioctl status 0
>      # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
>      [root@...alhost ~]# ./gup_test -LT -m 8192 -n 512
>      TAP version 13
>      1..1
>      # PIN_LONGTERM_BENCHMARK: Time: get:130727 put:31763 us#
>      ok 1 ioctl status 0
>      # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
> 
> Signed-off-by: Li Zhe <lizhe.67@...edance.com>
> ---
> Changelogs:
> 
> v1->v2:
> - Modify some unreliable code.
> - Update performance test data.
> 
> v1 patch: https://lore.kernel.org/all/20250530092351.32709-1-lizhe.67@bytedance.com/
> 
>   mm/gup.c | 37 +++++++++++++++++++++++++++++--------
>   1 file changed, 29 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 84461d384ae2..57fd324473a1 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2317,6 +2317,31 @@ static void pofs_unpin(struct pages_or_folios *pofs)
>   		unpin_user_pages(pofs->pages, pofs->nr_entries);
>   }
>   
> +static struct folio *pofs_next_folio(struct folio *folio,
> +				struct pages_or_folios *pofs, long *index_ptr)
> +{
> +	long i = *index_ptr + 1;
> +
> +	if (!pofs->has_folios) {

&& folio_test_large(folio)

To avoid all that for small folios.

> +		unsigned long start_pfn = folio_pfn(folio);> +		unsigned long end_pfn = start_pfn + folio_nr_pages(folio);

I guess both could be const

> +> +		for (; i < pofs->nr_entries; i++) {
> +			unsigned long pfn = page_to_pfn(pofs->pages[i]);
> +
> +			/* Is this page part of this folio? */
> +			if ((pfn < start_pfn) || (pfn >= end_pfn))


No need for the inner ()

> +				break;
> +		}
> +	}
> +
> +	if (unlikely(i == pofs->nr_entries))
> +		return NULL;
> +	*index_ptr = i;> +
> +	return pofs_get_folio(pofs, i);

We're now doing two "pofs->has_folios" checks. Maybe the compiler is
smart enough to figure that out.

> +}
> +
>   /*>    * Returns the number of collected folios. Return value is always >= 0.
>    */
> @@ -2324,16 +2349,12 @@ static void collect_longterm_unpinnable_folios(
>   		struct list_head *movable_folio_list,
>   		struct pages_or_folios *pofs)
>   {
> -	struct folio *prev_folio = NULL;
>   	bool drain_allow = true;
> -	unsigned long i;
> -
> -	for (i = 0; i < pofs->nr_entries; i++) {
> -		struct folio *folio = pofs_get_folio(pofs, i);
> +	long i = 0;
> +	struct folio *folio;

Please keep the reverse christmas tree where we have it. Why
the change from "unsigned long" -> "long" ?

>   
> -		if (folio == prev_folio)
> -			continue;
> -		prev_folio = folio;
> +	for (folio = pofs_get_folio(pofs, 0); folio;
> +			folio = pofs_next_folio(folio, pofs, &i)) {

Please indent as

for (folio = pofs_get_folio(pofs, 0); folio;
      folio = pofs_next_folio(folio, pofs, &i)) {

But the usage of "0" and "&i" is a bit suboptimal.

for (folio = pofs_get_folio(pofs, i); folio;
      folio = pofs_next_folio(folio, pofs, &i)) {

Might be better.

-- 
Cheers,

David / dhildenb