Re: [PATCH v2 10/11] memcg: allow a memcg with kmem charges to be destructed.

[Michal Hocko <mhocko@suse.cz>]

On Thu 09-08-12 17:01:18, 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.

This changes the semantic of memory.force_empty file because the usage
should be 0 on success but it will show kmem usage in fact now. I guess
it is inevitable with u+k accounting so you should be explicit about
that and also update the documentation. If some tests (I am not 100%
sure but I guess LTP) rely on that then they could be fixed by checking
the kmem limit as well.

> Signed-off-by: Glauber Costa <glommer@parallels.com>
> Acked-by: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> CC: Christoph Lameter <cl@linux.com>
> CC: Pekka Enberg <penberg@cs.helsinki.fi>
> CC: Michal Hocko <mhocko@suse.cz>
> CC: Johannes Weiner <hannes@cmpxchg.org>
> CC: Suleiman Souhlal <suleiman@google.com>
> ---
>  mm/memcontrol.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 3d30b79..7c1ea49 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -649,6 +649,11 @@ static void disarm_kmem_keys(struct mem_cgroup *memcg)
>  {
>  	if (test_bit(KMEM_ACCOUNTED_THIS, &memcg->kmem_accounted))
>  		static_key_slow_dec(&memcg_kmem_enabled_key);
> +	/*
> +	 * This check can't live in kmem destruction function,
> +	 * since the charges will outlive the cgroup
> +	 */
> +	WARN_ON(res_counter_read_u64(&memcg->kmem, RES_USAGE) != 0);
>  }
>  #else
>  static void disarm_kmem_keys(struct mem_cgroup *memcg)
> @@ -4005,6 +4010,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg, bool free_all)
>  	int node, zid, shrink;
>  	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
>  	struct cgroup *cgrp = memcg->css.cgroup;
> +	u64 usage;
>  
>  	css_get(&memcg->css);
>  
> @@ -4038,8 +4044,17 @@ move_account:
>  		mem_cgroup_end_move(memcg);
>  		memcg_oom_recover(memcg);
>  		cond_resched();
> +		/*
> +		 * Kernel memory may not necessarily be trackable to a specific
> +		 * process. So they are not migrated, and therefore we can't
> +		 * expect their value to drop to 0 here.
> +		 *
> +		 * having res filled up with kmem only is enough
> +		 */
> +		usage = res_counter_read_u64(&memcg->res, RES_USAGE) -
> +			res_counter_read_u64(&memcg->kmem, RES_USAGE);
>  	/* "ret" should also be checked to ensure all lists are empty. */
> -	} while (res_counter_read_u64(&memcg->res, RES_USAGE) > 0 || ret);
> +	} while (usage > 0 || ret);
>  out:
>  	css_put(&memcg->css);
>  	return ret;
> -- 
> 1.7.11.2
> 
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-- 
Michal Hocko
SUSE Labs

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