Re: [PATCH] mm, memcg: fix wrong mem cgroup protection

[Yafang Shao <laoar.shao@gmail.com>]

On Fri, Apr 24, 2020 at 5:13 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Thu, Apr 23, 2020 at 04:33:23PM +0100, Chris Down wrote:
> > Hi Yafang,
> >
> > I'm afraid I'm just as confused as Michal was about the intent of this patch.
> >
> > Can you please be more concise and clear about the practical ramifications
> > and demonstrate some pathological behaviour? I really can't visualise what's
> > wrong here from your explanation, and if I can't understand it as the person
> > who wrote this code, I am not surprised others are also confused :-)
> >
> > Or maybe Roman can try to explain, since he acked the previous patch? At
> > least to me, the emin/elow behaviour seems fairly non-trivial to reason
> > about right now.
>
> Hi Chris!
>
> So the thing is that emin/elow cached values are shared between global and
> targeted (caused by memory.max) reclaim. It's racy by design, but in general
> it should work ok, because in the end we'll reclaim or not approximately
> the same amount of memory.
>
> In the case which Yafang described, the emin value calculated in the process
> of the global reclaim leads to a slowdown of the targeted reclaim. It's not
> a tragedy, but not perfect too. It seems that the proposed patch makes it better,
> and as now I don't see any bad consequences.
>

Thanks for your explanation. Your explanation make the issue more clear.
If you don't mind, I will copy some of your comments to improve the commit log.



-- 
Thanks
Yafang