Re: [PATCH] memcg: Free spare array to avoid memory leak

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Mar 09, 2012 at 01:20:16PM +0900, KAMEZAWA Hiroyuki wrote:
> On Fri, 09 Mar 2012 12:07:32 +0800
> Sha Zhengju <handai.szj@gmail.com> wrote:
> 
> > On 03/09/2012 11:40 AM, KAMEZAWA Hiroyuki wrote:
> > > On Tue,  6 Mar 2012 20:13:24 +0800
> > > Sha Zhengju<handai.szj@gmail.com>  wrote:
> > >
> > >> From: Sha Zhengju<handai.szj@taobao.com>
> > >>
> > >> When the last event is unregistered, there is no need to keep the spare
> > >> array anymore. So free it to avoid memory leak.
> > >>
> > >> Signed-off-by: Sha Zhengju<handai.szj@taobao.com>
> > >>
> > >> ---
> > >>   mm/memcontrol.c |    6 ++++++
> > >>   1 files changed, 6 insertions(+), 0 deletions(-)
> > >>
> > >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > >> index 22d94f5..3c09a84 100644
> > >> --- a/mm/memcontrol.c
> > >> +++ b/mm/memcontrol.c
> > >> @@ -4412,6 +4412,12 @@ static void mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
> > >>   swap_buffers:
> > >>   	/* Swap primary and spare array */
> > >>   	thresholds->spare = thresholds->primary;
> > >> +	/* If all events are unregistered, free the spare array */
> > >> +	if (!new) {
> > >> +		kfree(thresholds->spare);
> > >> +		thresholds->spare = NULL;
> > >> +	}
> > >> +
> > > Could you clear thresholds->primary ? I don't like a pointer points to freed memory.
> > Do you meaning I should set a??thresholds->primary = NULLa?? i 1/4 ?
> > But the following rcu_assign_pointer will do this :
> > 
> > +	/* If all events are unregistered, free the spare array */
> > +	if (!new) {
> > +		kfree(thresholds->spare);
> > +		thresholds->spare = NULL;
> > +	}
> > +
> >   	rcu_assign_pointer(thresholds->primary, new);<---------*HERE*
> > 
> 
> Hm, ok.
> 
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> 
> BTW, can memory cgroup be destroyed while there are registered events ?

Yes, it can. All eventfds will be closed first. See cgroup_rmdir().

And here's possibility of leak. If we have an eventfd with >1 threasholds
attached to it, mem_cgroup_usage_unregister_event() will leave spare
not freed. And then we destroy cgroup...

Reviewed-by: Kirill A. Shutemov <kirill@shutemov.name>

-- 
 Kirill A. Shutemov

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