linux-kernel - Re: [PATCH v2] mm: Optimise put_pages

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Message-ID: <cb85b8a6-b82f-3054-5d76-57af018d6b2a@oracle.com>
Date:   Fri, 22 Oct 2021 16:26:59 -0700
From:   Anthony Yznaga <anthony.yznaga@...cle.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: Optimise put_pages_list()


On 10/7/21 12:21 PM, Matthew Wilcox (Oracle) wrote:
> Instead of calling put_page() one page at a time, pop pages off
> the list if their refcount was too high and pass the remainder to
> put_unref_page_list().  This should be a speed improvement, but I have
> no measurements to support that.  Current callers do not care about
> performance, but I hope to add some which do.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
> v2:
>   - Handle compound pages (Mel)
>   - Comment why we don't need to handle PageLRU
>   - Added call to __ClearPageWaiters(), matching that in release_pages()
>
>   mm/swap.c | 23 ++++++++++++++++-------
>   1 file changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index af3cad4e5378..9f334d503fd2 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -134,18 +134,27 @@ EXPORT_SYMBOL(__put_page);
>    * put_pages_list() - release a list of pages
>    * @pages: list of pages threaded on page->lru
>    *
> - * Release a list of pages which are strung together on page.lru.  Currently
> - * used by read_cache_pages() and related error recovery code.
> + * Release a list of pages which are strung together on page.lru.
>    */
>   void put_pages_list(struct list_head *pages)
>   {
> -	while (!list_empty(pages)) {
> -		struct page *victim;
> +	struct page *page, *next;
>   
> -		victim = lru_to_page(pages);
> -		list_del(&victim->lru);
> -		put_page(victim);
> +	list_for_each_entry_safe(page, next, pages, lru) {
> +		if (!put_page_testzero(page)) {
> +			list_del(&page->lru);
> +			continue;
> +		}


I know that compound pages are not currently passed to put_pages_list(),

but I assume the put_page_testzero() should only be done on the head

page similar to release_pages()?


Anthony

> +		if (PageHead(page)) {
> +			list_del(&page->lru);
> +			__put_compound_page(page);
> +			continue;
> +		}
> +		/* Cannot be PageLRU because it's passed to us using the lru */
> +		__ClearPageWaiters(page);
>   	}
> +
> +	free_unref_page_list(pages);
>   }
>   EXPORT_SYMBOL(put_pages_list);
>