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

[Roman Gushchin]

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