Re: [PATCH v3 3/4] mm: treat memory.low value inclusive

[Roman Gushchin <guro@fb.com>]

On Thu, Apr 05, 2018 at 03:45:26PM -0400, Johannes Weiner wrote:
> On Thu, Apr 05, 2018 at 07:59:20PM +0100, Roman Gushchin wrote:
> > If memcg's usage is equal to the memory.low value, avoid reclaiming
> > from this cgroup while there is a surplus of reclaimable memory.
> > 
> > This sounds more logical and also matches memory.high and memory.max
> > behavior: both are inclusive.
> 
> I was trying to figure out why we did it this way in the first place
> and found this patch:
> 
> commit 4e54dede38b45052a941bcf709f7d29f2e18174d
> Author: Michal Hocko <mhocko@suse.cz>
> Date:   Fri Feb 27 15:51:46 2015 -0800
> 
>     memcg: fix low limit calculation
>     
>     A memcg is considered low limited even when the current usage is equal to
>     the low limit.  This leads to interesting side effects e.g.
>     groups/hierarchies with no memory accounted are considered protected and
>     so the reclaim will emit MEMCG_LOW event when encountering them.
>     
>     Another and much bigger issue was reported by Joonsoo Kim.  He has hit a
>     NULL ptr dereference with the legacy cgroup API which even doesn't have
>     low limit exposed.  The limit is 0 by default but the initial check fails
>     for memcg with 0 consumption and parent_mem_cgroup() would return NULL if
>     use_hierarchy is 0 and so page_counter_read would try to dereference NULL.
>     
>     I suppose that the current implementation is just an overlook because the
>     documentation in Documentation/cgroups/unified-hierarchy.txt says:
>     
>       "The memory.low boundary on the other hand is a top-down allocated
>       reserve.  A cgroup enjoys reclaim protection when it and all its
>       ancestors are below their low boundaries"
>     
>     Fix the usage and the low limit comparision in mem_cgroup_low accordingly.
>     
> > @@ -5709,7 +5709,7 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
> >  	elow = min(elow, parent_elow * low_usage / siblings_low_usage);
> >  exit:
> >  	memcg->memory.elow = elow;
> > -	return usage < elow;
> > +	return usage <= elow;
> 
> So I think this needs to be usage && usage <= elow to not emit
> MEMCG_LOW events in case usage == elow == 0.

Perfect catch! Thanks, Johannes!

Updated version below.

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