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Sender: Minchan Kim <minchan.kim@gmail.com>
Date:   Tue, 28 Nov 2023 15:15:08 -0800
From:   Minchan Kim <minchan@kernel.org>
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        Liu Shixin <liushixin2@huawei.com>,
        Yu Zhao <yuzhao@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sachin Sant <sachinp@linux.ibm.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no
 swap space
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On Tue, Nov 28, 2023 at 03:05:29PM -0800, Yosry Ahmed wrote:
> On Tue, Nov 28, 2023 at 2:45 PM Minchan Kim <minchan@kernel.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@suse.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?

I think if the page was shared by MAP_SHARED, then, yes.
I think if the page was shared by MAP_PRIVATE but CoW(e.g., fork), then, no.