Re: [RFC][PATCH 3/4] memcg: fix for mem_cgroup_hierarchical_reclaim

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Fri, 9 Jan 2009 11:03:23 +0530, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> * Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> [2009-01-08 19:15:01]:
> 
> > If root_mem has no children, last_scaned_child is set to root_mem itself.
> > But after some children added to root_mem, mem_cgroup_get_next_node can
> > mem_cgroup_put the root_mem although root_mem has not been mem_cgroup_get.
> >
> 
> Good catch!
>  
Thanks :)

> > This patch fixes this behavior by:
> > - Set last_scanned_child to NULL if root_mem has no children or DFS search
> >   has returned to root_mem itself(root_mem is not a "child" of root_mem).
> >   Make mem_cgroup_get_first_node return root_mem in this case.
> >   There are no mem_cgroup_get/put for root_mem.
> > - Rename mem_cgroup_get_next_node to __mem_cgroup_get_next_node, and
> >   mem_cgroup_get_first_node to mem_cgroup_get_next_node.
> >   Make mem_cgroup_hierarchical_reclaim call only new mem_cgroup_get_next_node.
> >
> 
> How have you tested these changes? When I wrote up the patches, I did
> several tests to make sure that all nodes in the hierarchy are covered
> while reclaiming in order.
>  
I do something like:

  1. mount memcg (at/cgroup/memory)
  2. enable hierarchy (if testing use_hierarchy==1 case)
  3. mkdir /cgroup/memory/01
  4. run some programs in /cgroup/memory/01
  5. select the next directory to move to at random from 01/, 01/aa, 01/bb,
     02/, 02/aa and 02/bb.
  6. move all processes to next directory.
  7. remove the old directory if possible.
  8. wait for an random period.
  9. goto 5.

Before this patch, I got sometimes general protection fault, which seemed
to be caused by unexpected free of mem_cgroup (and reuse the area for
another purpose).


BTW, I think "mem_cgroup_put(mem->last_scanned_child)" is also needed
at mem_cgroup_destroy to prevent memory leak.

I'll update my patch later.


Thanks,
Daisuke Nishimura.

> > 
> > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > ---
> >  mm/memcontrol.c |   37 +++++++++++++++++++++----------------
> >  1 files changed, 21 insertions(+), 16 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 288e22c..dc38a0e 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -622,7 +622,7 @@ unsigned long mem_cgroup_isolate_pages(unsigned long nr_to_scan,
> >   * called with hierarchy_mutex held
> >   */
> >  static struct mem_cgroup *
> > -mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
> > +__mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
> >  {
> >  	struct cgroup *cgroup, *curr_cgroup, *root_cgroup;
> > 
> > @@ -644,8 +644,8 @@ mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
> >  visit_parent:
> >  	if (curr_cgroup == root_cgroup) {
> >  		mem_cgroup_put(curr);
> > -		curr = root_mem;
> > -		mem_cgroup_get(curr);
> > +		/* caller handles NULL case */
> > +		curr = NULL;
> >  		goto done;
> >  	}
> > 
> > @@ -668,7 +668,6 @@ visit_parent:
> >  	goto visit_parent;
> > 
> >  done:
> > -	root_mem->last_scanned_child = curr;
> >  	return curr;
> >  }
> > 
> > @@ -678,20 +677,29 @@ done:
> >   * that to reclaim free pages from.
> >   */
> >  static struct mem_cgroup *
> > -mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
> > +mem_cgroup_get_next_node(struct mem_cgroup *root_mem)
> >  {
> >  	struct cgroup *cgroup;
> >  	struct mem_cgroup *ret;
> >  	bool obsolete;
> > 
> > -	obsolete = mem_cgroup_is_obsolete(root_mem->last_scanned_child);
> > -
> >  	/*
> >  	 * Scan all children under the mem_cgroup mem
> >  	 */
> >  	mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
> > +
> > +	obsolete = mem_cgroup_is_obsolete(root_mem->last_scanned_child);
> > +
> >  	if (list_empty(&root_mem->css.cgroup->children)) {
> > -		ret = root_mem;
> > +		/*
> > +		 * root_mem might have children before and last_scanned_child
> > +		 * may point to one of them.
> > +		 */
> > +		if (root_mem->last_scanned_child) {
> > +			VM_BUG_ON(!obsolete);
> > +			mem_cgroup_put(root_mem->last_scanned_child);
> > +		}
> > +		ret = NULL;
> >  		goto done;
> >  	}
> > 
> > @@ -705,13 +713,13 @@ mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
> >  		ret = mem_cgroup_from_cont(cgroup);
> >  		mem_cgroup_get(ret);
> >  	} else
> > -		ret = mem_cgroup_get_next_node(root_mem->last_scanned_child,
> > +		ret = __mem_cgroup_get_next_node(root_mem->last_scanned_child,
> >  						root_mem);
> > 
> >  done:
> >  	root_mem->last_scanned_child = ret;
> >  	mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
> > -	return ret;
> > +	return (ret) ? ret : root_mem;
> >  }
> > 
> >  static bool mem_cgroup_check_under_limit(struct mem_cgroup *mem)
> > @@ -769,21 +777,18 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
> >  	if (!root_mem->use_hierarchy)
> >  		return ret;
> > 
> > -	next_mem = mem_cgroup_get_first_node(root_mem);
> > +	next_mem = mem_cgroup_get_next_node(root_mem);
> > 
> >  	while (next_mem != root_mem) {
> >  		if (mem_cgroup_is_obsolete(next_mem)) {
> > -			mem_cgroup_put(next_mem);
> > -			next_mem = mem_cgroup_get_first_node(root_mem);
> > +			next_mem = mem_cgroup_get_next_node(root_mem);
> >  			continue;
> >  		}
> >  		ret = try_to_free_mem_cgroup_pages(next_mem, gfp_mask, noswap,
> >  						   get_swappiness(next_mem));
> >  		if (mem_cgroup_check_under_limit(root_mem))
> >  			return 0;
> > -		mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
> > -		next_mem = mem_cgroup_get_next_node(next_mem, root_mem);
> > -		mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
> > +		next_mem = mem_cgroup_get_next_node(root_mem);
> >  	}
> >  	return ret;
> >  }
> > 
> 
> 
> Looks good to me, I need to test it though
> 
> Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
> 
> -- 
> 	Balbir

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