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

[Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>]

On Thu, 19 Apr 2012 16:54:50 +0800
Sha Zhengju <handai.szj-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> From: Sha Zhengju <handai.szj-3b8fjiQLQpfQT0dZR+AlfA@public.gmane.org>
> 
> 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?

> --- 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?


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.