Re: [PATCH] memcg, oom: lock mem_cgroup_print_oom_info

[Michal Hocko <mhocko@suse.cz>]

On Wed 11-12-13 14:23:18, David Rientjes wrote:
> On Wed, 11 Dec 2013, Michal Hocko wrote:
> 
> > mem_cgroup_print_oom_info uses a static buffer (memcg_name) to store the
> > name of the cgroup. This is not safe as pointed out by David Rientjes
> > because memcg oom is locked only for its hierarchy and nothing prevents
> > another parallel hierarchy to trigger oom as well and overwrite the
> > already in-use buffer.
> > 
> > This patch introduces oom_info_lock hidden inside mem_cgroup_print_oom_info
> > which is held throughout the function. It make access to memcg_name safe
> > and as a bonus it also prevents parallel memcg ooms to interleave their
> > statistics which would make the printed data hard to analyze otherwise.
> > 
> > Signed-off-by: Michal Hocko <mhocko@suse.cz>
> 
> Acked-by: David Rientjes <rientjes@google.com>

Thanks

> 
> > ---
> >  mm/memcontrol.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 28c9221b74ea..c72b03bf9679 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1647,13 +1647,13 @@ static void move_unlock_mem_cgroup(struct mem_cgroup *memcg,
> >   */
> >  void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
> >  {
> > -	struct cgroup *task_cgrp;
> > -	struct cgroup *mem_cgrp;
> >  	/*
> > -	 * Need a buffer in BSS, can't rely on allocations. The code relies
> > -	 * on the assumption that OOM is serialized for memory controller.
> > -	 * If this assumption is broken, revisit this code.
> > +	 * protects memcg_name and makes sure that parallel ooms do not
> > +	 * interleave
> 
> Parallel memcg oom kills can happen in disjoint memcg hierarchies, this 
> just prevents the printing of the statistics from interleaving.  I'm not 
> sure if that's clear from this comment.

What about this instead:
	* Protects memcg_name and makes sure that ooms from parallel
	* hierarchies do not interleave.
?
-- 
Michal Hocko
SUSE Labs

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