Re: [PATCH V3 1/3] mm: add swappiness=max arg to memory.reclaim for only anon reclaim

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  9 Apr 2025 15:06:18 +0800 Zhongkun He <hezhongkun.hzk@bytedance.com> 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.
> 
> So, we hope to add a new arg 'swappiness=max' in memory.reclaim where
> proactive memory reclaim only reclaims from anonymous folios when
> swappiness is set to max. The swappiness semantics from a user
> perspective remain unchanged.
> 
> For example, something like this:
> 
> echo "2M swappiness=max" > /sys/fs/cgroup/memory.reclaim
> 
> will perform reclaim on the rootcg with a swappiness setting of 'max' (a
> new mode) 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 or other file first logic 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 new
> mode 'max', means reclaiming just from anonymous folios both in traditional
> LRU and MGLRU.
> 
> ...
>
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1348,6 +1348,9 @@ The following nested keys are defined.
>  	same semantics as vm.swappiness applied to memcg reclaim with
>  	all the existing limitations and potential future extensions.
>  
> +	The valid range for swappiness is [0-200, max], setting
> +	swappiness=max exclusively reclaims anonymous memory.

Being able to assign either a number or a string feels a bit weird. 
Usually we use something like "-1" for a hack like this.  eg,
NUMA_NO_NODE.

Perhaps we just shouldn't overload swappiness like this.  Add a new
tunable (swappiness_mode?) which can override the swappiness setting.

I guess it doesn't matter much.  We're used to adding messy interfaces ;)