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From:   Chris Li <chrisl@kernel.org>
Date:   Thu, 23 Nov 2023 09:30:23 -0800
Message-ID: <CAF8kJuO=Y6frTxMbR92XhzuC8Z8ALDWFLq2Mj3t0j+C9YXOaJw@mail.gmail.com>
Subject: Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space
To:     Michal Hocko <mhocko@suse.com>
Cc:     Liu Shixin <liushixin2@huawei.com>, Yu Zhao <yuzhao@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yosry Ahmed <yosryahmed@google.com>,
        Huang Ying <ying.huang@intel.com>,
        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
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On Tue, Nov 21, 2023 at 5:00=E2=80=AFAM Michal Hocko <mhocko@suse.com> wrot=
e:
> > However, in swapcache_only mode, the scan count still increased when sc=
an
> > non-swapcache pages because there are large number of non-swapcache pag=
es
> > and rare swapcache pages in swapcache_only mode, and if the non-swapcac=
he
> > is skipped and do not count, the scan of pages in isolate_lru_folios() =
can
> > eventually lead to hung task, just as Sachin reported [2].
>
> I find this paragraph really confusing! I guess what you meant to say is
> that a real swapcache_only is problematic because it can end up not
> making any progress, correct?
>
> AFAIU you have addressed that problem by making swapcache_only anon LRU
> specific, right? That would be certainly more robust as you can still
> reclaim from file LRUs. I cannot say I like that because swapcache_only
> is a bit confusing and I do not think we want to grow more special

That is my feeling as well. I don't like to have too many special
purposes modes either. It makes the whole process much harder to
reason. The comment seems to suggest it is not effective in some
situations. I am wondering if we can address that situation more
directly without the special mode. At the same time I am not very
familiar with the reclaim code path yet. I need to learn more about
this problem space to articulate my thoughts better . I can dig in
more, I might ask a lot of silly questions.

Chris

> purpose reclaim types. Would it be possible/reasonable to instead put
> swapcache pages on the file LRU instead?
> --
> Michal Hocko
> SUSE Labs
>