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

[David Hildenbrand <david@redhat.com>]

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

-- 
Cheers,

David / dhildenb