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

[Sha Zhengju]

On 05/02/2012 05:03 AM, Andrew Morton wrote:
> On Thu, 19 Apr 2012 16:54:50 +0800
> Sha Zhengju<handai.szj@xxxxxxxxx>  wrote:
>
> > From: Sha Zhengju<handai.szj@xxxxxxxxxx>
> >
> > When the last event is unregistered, there is no need to keep the spare
> > array anymore. So free it to avoid memory leak.
> How serious is this leak?  Is there any way in which it can be used to
> consume unbounded amounts of memory?

While registering events, the ->primary will apply for a larger array to 
store
the new threshold info and the ->spare holds the old primary space.
But once unregistering event, the ->primary and ->spare pointer will be 
swapped
after updating thresholds info. So if we have an eventfd with many(>1) 
thresholds
attached to it, mem_cgroup_usage_unregister_event() will finally leave 
->spare
holding a large array and have no chance to be freed.

I hope it is clear.

> > --- 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;
> > +	}
> > +
> >   	rcu_assign_pointer(thresholds->primary, new);
> The resulting code is really quite convoluted.  Try to read through it
> and follow the handling of ->primary and ->spare.  Head spins.
>
> What is the protocol here?  If ->primary is NULL then ->spare must also
> be NULL?

To be simple:  if new(->primary) is NULL, it means we are unregistering
the last threshold and there is no need to keep ->spare any more.
So give the ->spare array a chance to be freed.

Thanks,
Sha

> I'll apply the patch, although I don't (yet) have sufficient info to
> know which kernels it should be applied to.  Perhaps someone could
> revisit this code and see if it can be made more straightforward.
>
> .

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/