Re: [PATCH v2] gup: optimize longterm pin_user_pages() for large folio

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  4 Jun 2025 11:15:36 +0800 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 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@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:4075 put:10792 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:130727 put:31763 us#
>     ok 1 ioctl status 0
>     # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

I see no READ_ONCE()s in the patch and I had to go off and read the v1
review to discover that the READ_ONCE is invoked in
page_folio()->_compound_head().  Please help us out by including such
details in the changelogs.

Is it credible that a humble READ_ONCE could yield a 3x improvement in
one case?  Why would this happen?