linux-kernel - Re: [PATCH RFC v2 2/9] mm: optimize do_wp_page() for fresh pages in local LRU pagevecs

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Date:   Wed, 26 Jan 2022 14:31:23 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>, linux-mm@...ck.org
Subject: Re: [PATCH RFC v2 2/9] mm: optimize do_wp_page() for fresh pages in
 local LRU pagevecs

On Wed, Jan 26, 2022 at 10:55:50AM +0100, David Hildenbrand wrote:
> diff --git a/mm/memory.c b/mm/memory.c
> index bcd3b7c50891..61d67ceef734 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3298,7 +3298,17 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  		 *
>  		 * PageKsm() doesn't necessarily raise the page refcount.
>  		 */
> -		if (PageKsm(page) || page_count(page) > 1 + PageSwapCache(page))
> +		if (PageKsm(page))
> +			goto copy;
> +		if (page_count(page) > 1 + PageSwapCache(page) + !PageLRU(page))
> +			goto copy;
> +		if (!PageLRU(page))
> +			/*
> +			 * Note: We cannot easily detect+handle references from
> +			 * remote LRU pagevecs or references to PageLRU() pages.
> +			 */
> +			lru_add_drain();
> +		if (page_count(page) > 1 + PageSwapCache(page))
>  			goto copy;

I worry we're starting to get too accurate here.  How about:

		if (PageKsm(page) || page_count(page) > 3)
			goto copy;
		if (!PageLRU(page))
			lru_add_drain();
		if (!trylock_page(page))
			goto copy;
...

>  		if (!trylock_page(page))
>  			goto copy;
> -- 
> 2.34.1
>