Re: [BUGFIX][PATCH mmotm] memcg: try_get_mem_cgroup_from_swapcache fix

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 23 Mar 2009 09:13:37 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:

> On Mon, 23 Mar 2009 00:10:15 +0530, Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> > * Daisuke Nishimura <d-nishimura@mtf.biglobe.ne.jp> [2009-03-23 00:02:38]:
> > 
> > > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > > 
> > > css_tryget can be called twice in !PageCgroupUsed case.
> > > 
> > > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > > ---
> > > This is a fix for cgroups-use-css-id-in-swap-cgroup-for-saving-memory-v5.patch
> > > 
> > >  mm/memcontrol.c |   10 ++++------
> > >  1 files changed, 4 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 5de6be9..55dea59 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -1027,9 +1027,11 @@ static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
> > >  	/*
> > >  	 * Used bit of swapcache is solid under page lock.
> > >  	 */
> > > -	if (PageCgroupUsed(pc))
> > > +	if (PageCgroupUsed(pc)) {
> > >  		mem = pc->mem_cgroup;
> > > -	else {
> > > +		if (mem && !css_tryget(&mem->css))
> > > +			mem = NULL;
> > > +	} else {
> > >  		ent.val = page_private(page);
> > >  		id = lookup_swap_cgroup(ent);
> > >  		rcu_read_lock();
> > > @@ -1038,10 +1040,6 @@ static struct mem_cgroup *try_get_mem_cgroup_from_swapcache(struct page *page)
> > >  			mem = NULL;
> > >  		rcu_read_unlock();
> > >  	}
> > > -	if (!mem)
> > > -		return NULL;
> > > -	if (!css_tryget(&mem->css))
> > > -		return NULL;
> > >  	return mem;
> > >  }
> > 
> > How did you detect the problem? Any test case/steps to reproduce the issue?
> > 
> I found this when rebasing my patch onto mmotm and reviewing it.
> 
> I suppose this bug can leads to an unremovable directory.
> 

I think I saw this in the weekend but couldn't find why ;(
Thank you!
-Kame

> 
> Thanks,
> Daisuke Nishimura.
> 

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