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

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Date:   Wed, 22 Nov 2023 09:55:06 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Yosry Ahmed <yosryahmed@...gle.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 Wed 22-11-23 14:57:24, Huang, Ying wrote:
> Yosry Ahmed <yosryahmed@...gle.com> writes:
> 
> > On Tue, Nov 21, 2023 at 10:41 PM Liu Shixin <liushixin2@...wei.com> wrote:
> >>
> >>
> >> On 2023/11/21 21:00, Michal Hocko wrote:
> >> > On Tue 21-11-23 17:06:24, Liu Shixin wrote:
> >> >
> >> > However, in swapcache_only mode, the scan count still increased when scan
> >> > non-swapcache pages because there are large number of non-swapcache pages
> >> > and rare swapcache pages in swapcache_only mode, and if the non-swapcache
> >> > 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?
> >> This paragraph is going to explain why checking swapcache_only after scan += nr_pages;
> >> >
> >> > 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
> >> > purpose reclaim types. Would it be possible/reasonable to instead put
> >> > swapcache pages on the file LRU instead?
> >> It looks like a good idea, but I'm not sure if it's possible. I can try it, is there anything to
> >> pay attention to?
> >
> > I think this might be more intrusive than we think. Every time a page
> > is added to or removed from the swap cache, we will need to move it
> > between LRUs. All pages on the anon LRU will need to go through the
> > file LRU before being reclaimed. I think this might be too big of a
> > change to achieve this patch's goal.
> 
> We need to identify swap cache pages on file LRU firstly.  It appears
> hard from the current definition of page flags.
> 
>   /* Filesystems */
>   PG_checked = PG_owner_priv_1,
> 
>   /* SwapBacked */
>   PG_swapcache = PG_owner_priv_1,       /* Swap page: swp_entry_t in private */

Checking along with folio_test_swapbacked would do the trick, right?

-- 
Michal Hocko
SUSE Labs