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Date:   Wed, 26 Jan 2022 14:25:25 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Liang Zhang <zhangliang5@huawei.com>, linux-mm@kvack.org,
        Nadav Amit <nadav.amit@gmail.com>
Subject: Re: [PATCH RFC v2 1/9] mm: optimize do_wp_page() for exclusive pages
 in the swapcache
Message-ID: <YfFZ1SruNMubhJLo@casper.infradead.org>
References: <20220126095557.32392-1-david@redhat.com>
 <20220126095557.32392-2-david@redhat.com>
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On Wed, Jan 26, 2022 at 10:55:49AM +0100, David Hildenbrand wrote:
> Liang Zhang reported [1] that the current COW logic in do_wp_page() is
> sub-optimal when it comes to swap+read fault+write fault of anonymous
> pages that have a single user, visible via a performance degradation in
> the redis benchmark. Something similar was previously reported [2] by
> Nadav with a simple reproducer.
> 
> Let's optimize for pages that have been added to the swapcache but only
> have an exclusive owner. Try removing the swapcache reference if there is
> hope that we're the exclusive user.
> 
> We will fail removing the swapcache reference in two scenarios:
> (1) There are additional swap entries referencing the page: copying
>     instead of reusing is the right thing to do.
> (2) The page is under writeback: theoretically we might be able to reuse
>     in some cases, however, we cannot remove the additional reference
>     and will have to copy.
> 
> Further, we might have additional references from the LRU pagevecs,
> which will force us to copy instead of being able to reuse. We'll try
> handling such references for some scenarios next. Concurrent writeback
> cannot be handled easily and we'll always have to copy.
> 
> While at it, remove the superfluous page_mapcount() check: it's
> implicitly covered by the page_count() for ordinary anon pages.
> 
> [1] https://lkml.kernel.org/r/20220113140318.11117-1-zhangliang5@huawei.com
> [2] https://lkml.kernel.org/r/0480D692-D9B2-429A-9A88-9BBA1331AC3A@gmail.com
> 
> Reported-by: Liang Zhang <zhangliang5@huawei.com>
> Reported-by: Nadav Amit <nadav.amit@gmail.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>