linux-kernel - Re: [PATCH] mm, memcg: reset low limit during memcg offlining

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Message-ID: <20170725120642.GA12635@castle.DHCP.thefacebook.com>
Date:   Tue, 25 Jul 2017 13:06:42 +0100
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     <linux-mm@...ck.org>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        <kernel-team@...com>, <cgroups@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, memcg: reset low limit during memcg offlining

On Tue, Jul 25, 2017 at 01:58:08PM +0200, Michal Hocko wrote:
> On Tue 25-07-17 12:40:47, Roman Gushchin wrote:
> > A removed memory cgroup with a defined low limit and some belonging
> > pagecache has very low chances to be freed.
> > 
> > If a cgroup has been removed, there is likely no memory pressure inside
> > the cgroup, and the pagecache is protected from the external pressure
> > by the defined low limit. The cgroup will be freed only after
> > the reclaim of all belonging pages. And it will not happen until
> > there are any reclaimable memory in the system. That means,
> > there is a good chance, that a cold pagecache will reside
> > in the memory for an undefined amount of time, wasting
> > system resources.
> > 
> > Fix this issue by zeroing memcg->low during memcg offlining.
> 
> Very well spotted! This goes all the way down to low limit inclusion
> AFAICS. I would be even tempted to mark it for stable because hiding
> some memory from reclaim basically indefinitely is not good. We might
> have been just lucky nobody has noticed that yet.

I believe it's because there are not so many actual low limit users,
and those who do, are using some offstream patches to mitigate this issue.

Thanks!

Roman