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Date:   Tue, 2 Jul 2019 13:03:18 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jonathan Corbet <corbet@lwn.net>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>, linux-mm@kvack.org,
        linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        Roman Gushchin <guro@fb.com>,
        Shakeel Butt <shakeelb@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH] mm, slab: Extend slab/shrink to shrink all the memcg
 caches
Message-Id: <20190702130318.39d187dc27dbdd9267788165@linux-foundation.org>
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On Tue,  2 Jul 2019 14:37:30 -0400 Waiman Long <longman@redhat.com> wrote:

> Currently, a value of '1" is written to /sys/kernel/slab/<slab>/shrink
> file to shrink the slab by flushing all the per-cpu slabs and free
> slabs in partial lists. This applies only to the root caches, though.
> 
> Extends this capability by shrinking all the child memcg caches and
> the root cache when a value of '2' is written to the shrink sysfs file.

Why?

Please fully describe the value of the proposed feature to or users. 
Always.

> 
> ...
>
> --- a/Documentation/ABI/testing/sysfs-kernel-slab
> +++ b/Documentation/ABI/testing/sysfs-kernel-slab
> @@ -429,10 +429,12 @@ KernelVersion:	2.6.22
>  Contact:	Pekka Enberg <penberg@cs.helsinki.fi>,
>  		Christoph Lameter <cl@linux-foundation.org>
>  Description:
> -		The shrink file is written when memory should be reclaimed from
> -		a cache.  Empty partial slabs are freed and the partial list is
> -		sorted so the slabs with the fewest available objects are used
> -		first.
> +		A value of '1' is written to the shrink file when memory should
> +		be reclaimed from a cache.  Empty partial slabs are freed and
> +		the partial list is sorted so the slabs with the fewest
> +		available objects are used first.  When a value of '2' is
> +		written, all the corresponding child memory cgroup caches
> +		should be shrunk as well.  All other values are invalid.

One would expect this to be a bitfield, like /proc/sys/vm/drop_caches. 
So writing 3 does both forms of shrinking.

Yes, it happens to be the case that 2 is a superset of 1, but what
about if we add "4"?