Re: [PATCH] mm, memcg: reset memcg's memory.{min, low} for reclaiming itself

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

On Sat, Dec 28, 2019 at 7:49 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Fri, Dec 27, 2019 at 07:43:53AM -0500, Yafang Shao wrote:
> > memory.{emin, elow} are set in mem_cgroup_protected(), and the values of
> > them won't be changed until next recalculation in this function. After
> > either or both of them are set, the next reclaimer to relcaim this memcg
> > may be a different reclaimer, e.g. this memcg is also the root memcg of
> > the new reclaimer, and then in mem_cgroup_protection() in get_scan_count()
> > the old values of them will be used to calculate scan count, that is not
> > proper. We should reset them to zero in this case.
> >
> > Here's an example of this issue.
> >
> >     root_mem_cgroup
> >          /
> >         A   memory.max=1024M memory.min=512M memory.current=800M
> >
> > Once kswapd is waked up, it will try to scan all MEMCGs, including
> > this A, and it will assign memory.emin of A with 512M.
> > After that, A may reach its hard limit(memory.max), and then it will
> > do memcg reclaim. Because A is the root of this reclaimer, so it will
> > not calculate its memory.emin. So the memory.emin is the old value
> > 512M, and then this old value will be used in
> > mem_cgroup_protection() in get_scan_count() to get the scan count.
> > That is not proper.
> >
> > Fixes: 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim")
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Chris Down <chris@chrisdown.name>
> > Cc: Roman Gushchin <guro@fb.com>
> > Cc: stable@vger.kernel.org
> > ---
> >  mm/memcontrol.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 601405b..bb3925d 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6287,8 +6287,17 @@ enum mem_cgroup_protection mem_cgroup_protected(struct mem_cgroup *root,
> >
> >       if (!root)
> >               root = root_mem_cgroup;
> > -     if (memcg == root)
> > +     if (memcg == root) {
> > +             /*
> > +              * Reset memory.(emin, elow) for reclaiming the memcg
> > +              * itself.
> > +              */
> > +             if (memcg != root_mem_cgroup) {
> > +                     memcg->memory.emin = 0;
> > +                     memcg->memory.elow = 0;
> > +             }
>
> I'm sorry, that didn't bring it from scratch, but I doubt that zeroing effecting
> protection is correct. Imagine a simple config: a large cgroup subtree with memory.max
> set on the top level. Reaching this limit doesn't mean that all protection
> configuration inside the tree can be ignored.
>

No, they won't be ignored.
Pls. see the logic in mem_cgroup_protected(), it will re-calculate all
its children's effective min and low.

> Instead we should respect memory.low/max set by a user on this level
> (look at the parent == root case), maybe clamped by memory.high/max.
>

Let's look at the parent == root case.
What if the parent is the root_mem_cgroup?
The memory.{emin, elow} of root_mem_cgroup is always 0 right ?
So what's your problem ?

Thanks
Yafang