Re: [RFC][PATCH] memcg: get/put parents at create/free

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

On Thu, 15 Jan 2009 16:54:53 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Thu, 15 Jan 2009 16:45:37 +0900
> Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> 
> > On Thu, 15 Jan 2009 13:38:14 +0900, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> > > On Thu, 15 Jan 2009 11:14:20 +0900, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> > > > > > > To handle the problem "parent may be obsolete",
> > > > > > > 
> > > > > > > call mem_cgroup_get(parent) at create()
> > > > > > > call mem_cgroup_put(parent) at freeing memcg.
> > > > > > >      (regardless of use_hierarchy.)
> > > > > > > 
> > > > > > > is clearer way to go, I think.
> > > > > > > 
> > > > > > > I wonder whether there is  mis-accounting problem or not..
> > > > > > > 
> > > hmm, after more consideration, although this patch can prevent the BUG,
> > > it can leak memsw accounting of parents because memsw of parents, which
> > > have been incremented by charge, does not decremented.
> > > 
> > > I'll try pet/put parent approach..
> > > Or any other good ideas ?
> > > 
> > I attach a tryial patch.
> > 
> > It has been working fine so far(for about 1 hour).
> > 
> > Thanks,
> > Daisuke Nishimura.
> > ===
> > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > 
> > mem_cgroup_get ensures that the memcg that has been got can be accessed
> > even after the directory has been removed, but it doesn't ensure that parents
> > of it can be accessed: parents might have been freed already by rmdir.
> > 
> > This causes a bug in case of use_hierarchy==1, because res_counter_uncharge
> > climb up the tree.
> > 
> > This patch tries to fix this probrem by getting parents at create, and
> > putting them at freeing.
> > 
> > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > ---
> >  mm/memcontrol.c |   33 ++++++++++++++++++++++++++++++++-
> >  1 files changed, 32 insertions(+), 1 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index fb62b43..b4aed07 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -202,6 +202,8 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
> >  
> >  static void mem_cgroup_get(struct mem_cgroup *mem);
> >  static void mem_cgroup_put(struct mem_cgroup *mem);
> > +static void mem_cgroup_get_parents(struct mem_cgroup *mem);
> > +static void mem_cgroup_put_parents(struct mem_cgroup *mem);
> >  
> >  static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
> >  					 struct page_cgroup *pc,
> > @@ -2185,10 +2187,38 @@ static void mem_cgroup_get(struct mem_cgroup *mem)
> >  
> >  static void mem_cgroup_put(struct mem_cgroup *mem)
> >  {
> > -	if (atomic_dec_and_test(&mem->refcnt))
> > +	if (atomic_dec_and_test(&mem->refcnt)) {
> > +		mem_cgroup_put_parents(mem);
> >  		__mem_cgroup_free(mem);
> > +	}
> > +}
> > +
> > +static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem)
> > +{
> > +	if (!mem->res.parent)
> > +		return NULL;
> > +	return mem_cgroup_from_res_counter(mem->res.parent, res);
> > +}
> > +
> > +static void mem_cgroup_get_parents(struct mem_cgroup *mem)
> > +{
> > +	struct mem_cgroup *parent = parent_mem_cgroup(mem);
> > +
> > +	while (parent) {
> > +		mem_cgroup_get(parent);
> > +		parent = parent_mem_cgroup(parent);
> > +	}
> >  }
> >  
> 
> does we have to add refcnt to all ancestors ?
> 
Ah, no need.

Just ensureing "a parent memcg is not freed while it has child memcg unfreed"
would be enough, because a parent is a child of parent of the parent.

This is the updated version.

Thanks,
Daisuke Nishimura.
===
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

mem_cgroup_get ensures that the memcg that has been got can be accessed
even after the directory has been removed, but it doesn't ensure that parents
of it can be accessed: parents might have been freed already by rmdir.

This causes a bug in case of use_hierarchy==1, because res_counter_uncharge
climb up the tree.

This patch tries to fix this probrem by getting the parent at create, and
putting it at freeing.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 mm/memcontrol.c |   29 ++++++++++++++++++++++++++++-
 1 files changed, 28 insertions(+), 1 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fb62b43..a80ba68 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -202,6 +202,8 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
 
 static void mem_cgroup_get(struct mem_cgroup *mem);
 static void mem_cgroup_put(struct mem_cgroup *mem);
+static void mem_cgroup_get_parent(struct mem_cgroup *mem);
+static void mem_cgroup_put_parent(struct mem_cgroup *mem);
 
 static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
 					 struct page_cgroup *pc,
@@ -2185,10 +2187,34 @@ static void mem_cgroup_get(struct mem_cgroup *mem)
 
 static void mem_cgroup_put(struct mem_cgroup *mem)
 {
-	if (atomic_dec_and_test(&mem->refcnt))
+	if (atomic_dec_and_test(&mem->refcnt)) {
+		mem_cgroup_put_parent(mem);
 		__mem_cgroup_free(mem);
+	}
+}
+
+static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem)
+{
+	if (!mem->res.parent)
+		return NULL;
+	return mem_cgroup_from_res_counter(mem->res.parent, res);
+}
+
+static void mem_cgroup_get_parent(struct mem_cgroup *mem)
+{
+	struct mem_cgroup *parent = parent_mem_cgroup(mem);
+
+	if (parent)
+		mem_cgroup_get(parent);
 }
 
+static void mem_cgroup_put_parent(struct mem_cgroup *mem)
+{
+	struct mem_cgroup *parent = parent_mem_cgroup(mem);
+
+	if (parent)
+		mem_cgroup_put(parent);
+}
 
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 static void __init enable_swap_cgroup(void)
@@ -2237,6 +2263,7 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 	if (parent)
 		mem->swappiness = get_swappiness(parent);
 	atomic_set(&mem->refcnt, 1);
+	mem_cgroup_get_parent(mem);
 	return &mem->css;
 free_out:
 	__mem_cgroup_free(mem);

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