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From: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Subject: Re: [PATCH 0/3] memcg: Slow down swap allocation as the available
 space gets depleted
Date: Wed, 22 Apr 2020 10:15:14 -0400
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To: Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Jakub Kicinski <kuba-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Linux MM <linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>, Kernel Team <kernel-team-b10kYP2dOMg@public.gmane.org>, Chris Down <chris-6Bi1550iOqEnzZ6mRAm98g@public.gmane.org>, Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>

On Wed, Apr 22, 2020 at 03:26:32PM +0200, Michal Hocko wrote:
> That being said I believe our discussion is missing an important part.
> There is no description of the swap.high semantic. What can user expect
> when using it?

Good point, we should include that in cgroup-v2.rst. How about this?

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index bcc80269bb6a..49e8733a9d8a 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1370,6 +1370,17 @@ PAGE_SIZE multiple when read back.
 	The total amount of swap currently being used by the cgroup
 	and its descendants.
 
+  memory.swap.high
+	A read-write single value file which exists on non-root
+	cgroups.  The default is "max".
+
+	Swap usage throttle limit.  If a cgroup's swap usage exceeds
+	this limit, allocations inside the cgroup will be throttled.
+
+	This slows down expansion of the group's memory footprint as
+	it runs out of assigned swap space. Compare to memory.swap.max,
+	which stops swapping abruptly and can provoke kernel OOM kills.
+
   memory.swap.max
 	A read-write single value file which exists on non-root
 	cgroups.  The default is "max".