linux-kernel - Re: [PATCH] mm: Free unused swap cache page in write protection fault handler

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Date:   Wed, 13 Jan 2021 11:08:56 +0800
From:   huang ying <huang.ying.caritas@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Huang Ying <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...riel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH] mm: Free unused swap cache page in write protection fault handler

On Wed, Jan 13, 2021 at 10:47 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, Jan 12, 2021 at 6:43 PM Huang Ying <ying.huang@...el.com> wrote:
> >
> > So in this patch, at the end of wp_page_copy(), the old unused swap
> > cache page will be tried to be freed.
>
> I'd much rather free it later when needed, rather than when you're in
> a COW section.

Unused swap cache isn't unused file cache.  Nobody can reuse them
directly before freeing them firstly.  It will make COW a little
faster via keeping them.  But I think the overhead to free them isn't
high.  While keeping them in system will confuse users (users will
expect file cache to use free memory, but not expect unused swap cache
to use much free memory), make the swap space more fragmented, and add
system overall overhead (scanning LRU list, etc.).

Best Regards,
Huang, Ying