Re: [PATCH] gup: optimize longterm pin_user_pages() for large folio

[lizhe.67@bytedance.com]

On Fri, 30 May 2025 13:31:26 +0200, david@redhat.com wrote:

> On 30.05.25 11:23, lizhe.67@bytedance.com wrote:
> > From: Li Zhe <lizhe.67@bytedance.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 number of if-else
> > branches and 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 75%
> > for large folio with pagesize=2M. For normal page, we have only observed
> > a very slight degradation in performance.
> > 
> > Without this patch:
> > 
> >      [root@localhost ~] ./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@localhost ~]# ./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@localhost ~] ./gup_test -HL -m 8192 -n 512
> >      TAP version 13
> >      1..1
> >      # PIN_LONGTERM_BENCHMARK: Time: get:3386 put:10844 us#
> >      ok 1 ioctl status 0
> >      # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
> >      [root@localhost ~]# ./gup_test -LT -m 8192 -n 512
> >      TAP version 13
> >      1..1
> >      # PIN_LONGTERM_BENCHMARK: Time: get:131652 put:31393 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@bytedance.com>
> > ---
> >   mm/gup.c | 31 +++++++++++++++++++++++--------
> >   1 file changed, 23 insertions(+), 8 deletions(-)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 84461d384ae2..8c11418036e2 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2317,6 +2317,25 @@ 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;
> > +	unsigned long nr_pages = folio_nr_pages(folio);
> > +
> > +	if (!pofs->has_folios)
> > +		while ((i < pofs->nr_entries) &&
> > +			/* Is this page part of this folio? */
> > +			(folio_page_idx(folio, pofs->pages[i]) < nr_pages))
> 
> passing in a page that does not belong to the folio looks shaky and not 
> future proof.
> 
> folio_page() == folio
> 
> is cleaner

Yes, this approach is cleaner. However, when obtaining a folio
corresponding to a page through the page_folio() interface,
READ_ONCE() is used internally to read from memory, which results
in the performance of pin_user_pages() being worse than before.
Could you please suggest an alternative approach to address this
problem?

Thanks,
Zhe