[Devel] Re: [PATCH 10/11] memcg: allow a memcg with kmem charges to be destructed.

[Tejun Heo]

On Mon, Jun 25, 2012 at 06:15:27PM +0400, Glauber Costa wrote:
> Because the ultimate goal of the kmem tracking in memcg is to
> track slab pages as well, we can't guarantee that we'll always
> be able to point a page to a particular process, and migrate
> the charges along with it - since in the common case, a page
> will contain data belonging to multiple processes.
> 
> Because of that, when we destroy a memcg, we only make sure
> the destruction will succeed by discounting the kmem charges
> from the user charges when we try to empty the cgroup.
> 
> Signed-off-by: Glauber Costa <glommer at parallels.com>
> CC: Christoph Lameter <cl at linux.com>
> CC: Pekka Enberg <penberg at cs.helsinki.fi>
> CC: Michal Hocko <mhocko at suse.cz>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> CC: Johannes Weiner <hannes at cmpxchg.org>
> CC: Suleiman Souhlal <suleiman at google.com>
> ---
>  mm/memcontrol.c |   10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index a6a440b..bb9b6fe 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -598,6 +598,11 @@ static void disarm_kmem_keys(struct mem_cgroup *memcg)
>  {
>  	if (test_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted))
>  		static_key_slow_dec(&mem_cgroup_kmem_enabled_key);
> +	/*
> +	 * This check can't live in kmem destruction function,
> +	 * since the charges will outlive the cgroup
> +	 */
> +	BUG_ON(res_counter_read_u64(&memcg->kmem, RES_USAGE) != 0);

WARN_ON() please.  Misaccounted kernel usually is better than dead
kernel.

Thanks.

-- 
tejun