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

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Message-ID: <20210113031142.GM35215@casper.infradead.org>
Date:   Wed, 13 Jan 2021 03:11:42 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     huang ying <huang.ying.caritas@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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 11:08:56AM +0800, huang ying wrote:
> 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.).

Couldn't we just move it to the tail of the LRU list so it's reclaimed
first?  Or is locking going to be a problem here?