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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 18 May 2021 17:17:21 -1000
Message-ID: <CAHk-=wig4SQjBLtmFODs6oXukwrO0o5CPUVzxT2UvJM9jvwSiA@mail.gmail.com>
Subject: Re: [PATCH] mm: move idle swap cache pages to the tail of LRU after COW
To:     Huang Ying <ying.huang@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Peter Xu <peterx@redhat.com>, Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@surriel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Michal Hocko <mhocko@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tim Chen <tim.c.chen@intel.com>
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On Tue, May 18, 2021 at 3:33 PM Huang Ying <ying.huang@intel.com> wrote:
>
> With commit 09854ba94c6a ("mm: do_wp_page() simplification"), after
> COW, the idle swap cache (neither the page nor the corresponding swap
> entry is mapped by any process) will be left at the original position
> in the LRU list.  While it may be in the active list or the head of
> the inactive list, so that vmscan may take more overhead or time to
> reclaim these actually unused pages.

This looks sensible to me (and numbers talk!), but as Rik says, it
would probably be a good idea to move the trylock_page()/unlock_page()
into try_to_free_idle_swapcache(), and that would make the calling
side a whole lot cleaner and easier to read.

                    Linus