linux-kernel - Re: [PATCH v3] gup: optimize longterm pin_user

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Message-ID: <9b04871b-33fb-42cb-840b-88fdb6b93c48@redhat.com>
Date: Thu, 5 Jun 2025 09:51:06 +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 v3] gup: optimize longterm pin_user_pages() for large
 folio

On 05.06.25 05:34, 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 collect_longterm_unpinnable_folios() by reducing
> the using of READ_ONCE() invoked in
> pofs_get_folio()->page_folio()->_compound_head(). Also, we can simplify
> the logic of collect_longterm_unpinnable_folios(). Instead of comparing
> with prev_folio after calling pofs_get_folio(), we can check whether the
> next page is within the same folio.
> 
> The performance test results, based on v6.15, obtained through the
> gup_test tool from the kernel source tree are as follows. We achieve an
> improvement of over 66% for large folio with pagesize=2M. For small folio,
> 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:14391 put:10858 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:130538 put:31676 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:4867 put:10516 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:131798 put:31328 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:
> 
> v2->v3:
> - Update performance test data based on v6.15.
> - Refine the description of the optimization approach in commit message.
> - Fix some issues of code formatting.
> - Fine-tune the conditions for entering the optimization path.
> 
> v1->v2:
> - Modify some unreliable code.
> - Update performance test data.
> 
> v2 patch: https://lore.kernel.org/all/20250604031536.9053-1-lizhe.67@bytedance.com/
> 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..9fbe3592b5fc 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)

^ use two tabs here

> +{
> +	long i = *index_ptr + 1;
> +
> +	if (!pofs->has_folios && folio_test_large(folio)) {
> +		const unsigned long start_pfn = folio_pfn(folio);
> +		const unsigned long end_pfn = start_pfn + folio_nr_pages(folio);
> +
> +		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)
> +				break;
> +		}
> +	}
> +
> +	if (unlikely(i == pofs->nr_entries))
> +		return NULL;
> +	*index_ptr = i;
> +
> +	return pofs_get_folio(pofs, i);
> +}
> +
>   /*
>    * 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);
> +	struct folio *folio;
> +	long i = 0;
>   
> -		if (folio == prev_folio)
> -			continue;
> -		prev_folio = folio;
> +	for (folio = pofs_get_folio(pofs, i); folio;
> +			folio = pofs_next_folio(folio, pofs, &i)) {

As discussed, align both "folios" (using tabs and then spaces)

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb