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

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Mar 14, 2025 at 10:27:57AM +0100, Michal Hocko wrote:
> 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.

I'm guess I'm not seeing the "under-defined" part. cache_trim_mode is
there to make sure a streaming file access pattern doesn't cause
swapping. He has a special usecase to override cache_trim_mode when he
knows a large amount of anon is going cold. There is no way we can
generally remove it from proactive reclaim.