linux-kernel - Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space

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Date:   Tue, 28 Nov 2023 15:05:29 -0800
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Michal Hocko <mhocko@...e.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sachin Sant <sachinp@...ux.ibm.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space

On Tue, Nov 28, 2023 at 2:45 PM Minchan Kim <minchan@...nel.org> wrote:
>
> On Tue, Nov 28, 2023 at 11:16:04AM +0100, Michal Hocko wrote:
> > On Tue 28-11-23 09:31:06, Huang, Ying wrote:
> > > Michal Hocko <mhocko@...e.com> writes:
> > [...]
> > > > Right. On the other hand we could be more aggressive when dropping the
> > > > swapcache. Is there any actual reason why we cannot try to folio_free_swap
> > > > even when mem_cgroup_swap_full == F?
> > >
> > > If there are plenty free space in swap device, why not take advantage of
> > > it?
> >
> > Maybe a stupid question but what is the advantage of keeping around in
> > the swap cache?
>
> If the page is shared, we avoids addtional IO to bring them back so
> swap cache.

I think this case is actually necessary for correctness, not just to
avoid additional IO. Otherwise subsequent swapins will create new
copies of the page, right?

>
> If the page is still clean on reclaim moment since swap-in, VM doesn't
> need to write them out to the swap disk since original data is already
> there.