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

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Message-ID: <20250605082305.5280-1-lizhe.67@bytedance.com>
Date: Thu,  5 Jun 2025 16:23:05 +0800
From: lizhe.67@...edance.com
To: david@...hat.com
Cc: akpm@...ux-foundation.org,
	dev.jain@....com,
	jgg@...pe.ca,
	jhubbard@...dia.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	lizhe.67@...edance.com,
	muchun.song@...ux.dev,
	peterx@...hat.com
Subject: Re: [PATCH v3] gup: optimize longterm pin_user_pages() for large folio

On Thu, 5 Jun 2025 09:51:06 +0200, david@...hat.com wrote:

> 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>

Thank you very much for your review. I will fix the issue in v4 patch.

Thanks,
Zhe