Re: [PATCH v2 2/3] mm, memcg: propagate memory effective protection on setting memory.min/low

[Roman Gushchin <guro@fb.com>]

On Tue, Jun 12, 2018 at 11:52:42AM -0400, Johannes Weiner wrote:
> On Mon, Jun 11, 2018 at 10:54:17AM -0700, Roman Gushchin wrote:
> > Explicitly propagate effective memory min/low values down by the tree.
> > 
> > If there is the global memory pressure, it's not really necessary.
> > Effective memory guarantees will be propagated automatically as we
> > traverse memory cgroup tree in the reclaim path.
> > 
> > But if there is no global memory pressure, effective memory protection
> > still matters for local (memcg-scoped) memory pressure.  So, we have to
> > update effective limits in the subtree, if a user changes memory.min and
> > memory.low values.
> > 
> > Link: http://lkml.kernel.org/r/20180522132528.23769-1-guro@fb.com
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> > Cc: Greg Thelen <gthelen@google.com>
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: Shuah Khan <shuah@kernel.org>
> > Signed-off-by: Andrew Morton <akpm@linuxfoundation.org>
> > ---
> >  mm/memcontrol.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 5a3873e9d657..485df6f63d26 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5084,7 +5084,7 @@ static int memory_min_show(struct seq_file *m, void *v)
> >  static ssize_t memory_min_write(struct kernfs_open_file *of,
> >  				char *buf, size_t nbytes, loff_t off)
> >  {
> > -	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> > +	struct mem_cgroup *iter, *memcg = mem_cgroup_from_css(of_css(of));
> >  	unsigned long min;
> >  	int err;
> >  
> > @@ -5095,6 +5095,11 @@ static ssize_t memory_min_write(struct kernfs_open_file *of,
> >  
> >  	page_counter_set_min(&memcg->memory, min);
> >  
> > +	rcu_read_lock();
> > +	for_each_mem_cgroup_tree(iter, memcg)
> > +		mem_cgroup_protected(NULL, iter);
> > +	rcu_read_unlock();
> 
> I'm not quite following. mem_cgroup_protected() is a just-in-time
> query that depends on the groups' usage. How does it make sense to run
> this at the time the limit is set?

mem_cgroup_protected() emulates memory pressure to propagate
effective memory guarantee values.
> 
> Also, why is target reclaim different from global reclaim here? We
> have all the information we need, even if we don't start at the
> root_mem_cgroup. If we enter target reclaim against a specific cgroup,
> yes, we don't know the elow it receives from its parents. What we *do*
> know, though, is that it hit its own hard limit. What is happening
> higher up that group doesn't matter for the purpose of protection.
> 
> I.e. it seems to me that instead of this patch we should be treating
> the reclaim root and its first-level children the same way we treat
> root_mem_cgroup and top-level cgroups: no protection for the root,
> first children use their low setting as the elow, all descendants get
> the proportional low-usage distribution.
> 

Ok, we can keep it this way. We can have some races between the global
and targeted reclaim, but it's fine.

Andrew,
can you, please, drop these patches from the mm tree:
  selftests: cgroup: add test for memory.low corner cases
  mm, memcg: don't skip memory guarantee calculations
  mm, memcg: propagate memory effective protection on setting memory.min/low

The null pointer fix ("b2c21aa3690a mm: fix null pointer dereference in mem_cgroup_protected")
should be kept and merged asap.

Thank you!