linux-kernel - Re: [PATCH V2] mm: vmscan: skip the file folios in proactive reclaim if swappiness is MAX

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Message-ID: <Z9P2nZ6b75FRMhCp@tiehlicka>
Date: Fri, 14 Mar 2025 10:27:57 +0100
From: Michal Hocko <mhocko@...e.com>
To: Zhongkun He <hezhongkun.hzk@...edance.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, muchun.song@...ux.dev,
	yosry.ahmed@...ux.dev, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mm: vmscan: skip the file folios in proactive reclaim
 if swappiness is MAX

On Fri 14-03-25 09:52:45, Michal Hocko wrote:
> On Fri 14-03-25 11:33:50, Zhongkun He wrote:
> > With this patch 'commit <68cd9050d871> ("mm: add swappiness= arg to
> > memory.reclaim")', we can submit an additional swappiness=<val> argument
> > to memory.reclaim. It is very useful because we can dynamically adjust
> > the reclamation ratio based on the anonymous folios and file folios of
> > each cgroup. For example,when swappiness is set to 0, we only reclaim
> > from file folios.
> > 
> > However,we have also encountered a new issue: when swappiness is set to
> > the MAX_SWAPPINESS, it may still only reclaim file folios. This is due
> > to the knob of cache_trim_mode, which depends solely on the ratio of
> > inactive folios, regardless of whether there are a large number of cold
> > folios in anonymous folio list.
> > 
> > So, we hope to add a new control logic where proactive memory reclaim only
> > reclaims from anonymous folios when swappiness is set to MAX_SWAPPINESS.
> > For example, something like this:
> > 
> > echo "2M swappiness=200" > /sys/fs/cgroup/memory.reclaim
> > 
> > will perform reclaim on the rootcg with a swappiness setting of 200 (max
> > swappiness) regardless of the file folios. Users have a more comprehensive
> > view of the application's memory distribution because there are many
> > metrics available. For example, if we find that a certain cgroup has a
> > large number of inactive anon folios, we can reclaim only those and skip
> > file folios, because with the zram/zswap, the IO tradeoff that
> > cache_trim_mode is making doesn't hold - file refaults will cause IO,
> > whereas anon decompression will not.
> > 
> > With this patch, the swappiness argument of memory.reclaim has a more
> > precise semantics: 0 means reclaiming only from file pages, while 200
> > means reclaiming just from anonymous pages.
> 
> Haven't you said you will try a slightly different approach and always
> bypass LRU balancing heuristics for pro-active reclaim and swappiness
> provided? What has happened with that?

I have just noticed that you have followed up [1] with a concern that
using swappiness in the whole min-max range without any heuristics turns
out to be harder than just relying on the min and max as extremes.
What seems to be still missing (or maybe it is just me not seeing that)
is why should we only enforce those extreme ends of the range and still
preserve under-defined semantic for all other swappiness values in the
pro-active reclaim.

[1] https://lore.kernel.org/all/CACSyD1OHD8oXQcQmi1D9t2f5oeMVDvCQnYZUMQTGbqBz4YYKLQ@mail.gmail.com/T/#u
-- 
Michal Hocko
SUSE Labs