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SubjectRe: [PATCH 2/4] mm/gup: decrement head page once for group of subpages
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On 2/3/21 2:00 PM, Joao Martins wrote:
> Rather than decrementing the head page refcount one by one, we
> walk the page array and checking which belong to the same
> compound_head. Later on we decrement the calculated amount
> of references in a single write to the head page. To that
> end switch to for_each_compound_head() does most of the work.
>
> set_page_dirty() needs no adjustment as it's a nop for
> non-dirty head pages and it doesn't operate on tail pages.
>
> This considerably improves unpinning of pages with THP and
> hugetlbfs:
>
> - THP
> gup_test -t -m 16384 -r 10 [-L|-a] -S -n 512 -w
> PIN_LONGTERM_BENCHMARK (put values): ~87.6k us -> ~23.2k us
>
> - 16G with 1G huge page size
> gup_test -f /mnt/huge/file -m 16384 -r 10 [-L|-a] -S -n 512 -w
> PIN_LONGTERM_BENCHMARK: (put values): ~87.6k us -> ~27.5k us
>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
> mm/gup.c | 29 +++++++++++------------------
> 1 file changed, 11 insertions(+), 18 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 4f88dcef39f2..971a24b4b73f 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -270,20 +270,15 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
> bool make_dirty)
> {
> unsigned long index;
> -
> - /*
> - * TODO: this can be optimized for huge pages: if a series of pages is
> - * physically contiguous and part of the same compound page, then a
> - * single operation to the head page should suffice.
> - */

Great to see this TODO (and the related one below) finally done!

Everything looks correct here.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>


thanks,
--
John Hubbard
NVIDIA

> + struct page *head;
> + unsigned int ntails;
>
> if (!make_dirty) {
> unpin_user_pages(pages, npages);
> return;
> }
>
> - for (index = 0; index < npages; index++) {
> - struct page *page = compound_head(pages[index]);
> + for_each_compound_head(index, pages, npages, head, ntails) {
> /*
> * Checking PageDirty at this point may race with
> * clear_page_dirty_for_io(), but that's OK. Two key
> @@ -304,9 +299,9 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
> * written back, so it gets written back again in the
> * next writeback cycle. This is harmless.
> */
> - if (!PageDirty(page))
> - set_page_dirty_lock(page);
> - unpin_user_page(page);
> + if (!PageDirty(head))
> + set_page_dirty_lock(head);
> + put_compound_head(head, ntails, FOLL_PIN);
> }
> }
> EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
> @@ -323,6 +318,8 @@ EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
> void unpin_user_pages(struct page **pages, unsigned long npages)
> {
> unsigned long index;
> + struct page *head;
> + unsigned int ntails;
>
> /*
> * If this WARN_ON() fires, then the system *might* be leaking pages (by
> @@ -331,13 +328,9 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
> */
> if (WARN_ON(IS_ERR_VALUE(npages)))
> return;
> - /*
> - * TODO: this can be optimized for huge pages: if a series of pages is
> - * physically contiguous and part of the same compound page, then a
> - * single operation to the head page should suffice.
> - */
> - for (index = 0; index < npages; index++)
> - unpin_user_page(pages[index]);
> +
> + for_each_compound_head(index, pages, npages, head, ntails)
> + put_compound_head(head, ntails, FOLL_PIN);
> }
> EXPORT_SYMBOL(unpin_user_pages);
>
>

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