On Wed 27-11-13 19:46:01, Vladimir Davydov wrote:
> We should start kmem accounting for a memory cgroup only after both its
> kmem limit is set (KMEM_ACCOUNTED_ACTIVE) and related call sites are
> patched (KMEM_ACCOUNTED_ACTIVATED).
This should be vice-versa, no? ACTIVE is set after
static_key_slow_inc(&memcg_kmem_enabled_key) AFAICS.
> Currently memcg_can_account_kmem() allows kmem accounting even if only
> one of the conditions is true.
> Fix it.
It would be nice to describe, what is the actual problem here. I assume
this is a charge vs. enable race. Let me try
So we have KMEM_ACCOUNTED_ACTIVATED (set by memcg_update_cache_sizes)
but the static key is not enabled yet (so KMEM_ACCOUNTED_ACTIVE is not
set yet). memcg_can_account_kmem is called from 2 contexts during charge
- memcg_kmem_get_cache via __memcg_kmem_get_cache
- memcg_kmem_newpage_charge via __memcg_kmem_newpage_charge
both of them start by checking memcg_kmem_enabled which is our
static key before memcg_can_account_kmem. This would suggest that
static_key+ACTIVE check memcg_can_account_kmem is sufficient. No?
That being said the proposed change is not incorrect it just doesn't
seem to _fix_ anything. I would much rather see a comprehensive
documentation of the whole enabling workflow. E.g. why do we need
ACTIVATED at all? Nobody seem to care in the code...
> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: Balbir Singh <bsingharora@xxxxxxxxx>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> ---
> mm/memcontrol.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f1a0ae6..40efb9d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2956,7 +2956,8 @@ static DEFINE_MUTEX(set_limit_mutex);
> static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
> {
> return !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
> - (memcg->kmem_account_flags & KMEM_ACCOUNTED_MASK);
> + (memcg->kmem_account_flags & KMEM_ACCOUNTED_MASK) ==
> + KMEM_ACCOUNTED_MASK;
> }
>
> /*
> --
> 1.7.10.4
>
--
Michal Hocko
SUSE Labs
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