[PATCH] memcg: fix kmem_account_flags check in memcg_can_account_kmem()

[PATCH] memcg: fix kmem_account_flags check in memcg_can_account_kmem()

[Vladimir Davydov]

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). Currently memcg_can_account_kmem()
allows kmem accounting even if only one of the conditions is true.
Fix it.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 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

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[PATCH] memcg: make memcg_update_cache_sizes() static

[Vladimir Davydov]

This function is not used outside of memcontrol.c so make it static.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 40efb9d..b20b915 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3084,7 +3084,7 @@ int memcg_cache_id(struct mem_cgroup *memcg)
  * But when we create a new cache, we can call this as well if its parent
  * is kmem-limited. That will have to hold set_limit_mutex as well.
  */
-int memcg_update_cache_sizes(struct mem_cgroup *memcg)
+static int memcg_update_cache_sizes(struct mem_cgroup *memcg)
 {
 	int num, ret;
 
-- 
1.7.10.4

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Re: [PATCH] memcg: make memcg_update_cache_sizes() static

[Michal Hocko]

On Wed 27-11-13 19:46:02, Vladimir Davydov wrote:
> This function is not used outside of memcontrol.c so make it static.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: Balbir Singh <bsingharora@gmail.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
>  mm/memcontrol.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 40efb9d..b20b915 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3084,7 +3084,7 @@ int memcg_cache_id(struct mem_cgroup *memcg)
>   * But when we create a new cache, we can call this as well if its parent
>   * is kmem-limited. That will have to hold set_limit_mutex as well.
>   */
> -int memcg_update_cache_sizes(struct mem_cgroup *memcg)
> +static int memcg_update_cache_sizes(struct mem_cgroup *memcg)
>  {
>  	int num, ret;
>  
> -- 
> 1.7.10.4
> 
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-- 
Michal Hocko
SUSE Labs

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Re: [PATCH] memcg: fix kmem_account_flags check in memcg_can_account_kmem()

[Michal Hocko]

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@parallels.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: Balbir Singh <bsingharora@gmail.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  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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Re: [PATCH] memcg: fix kmem_account_flags check in memcg_can_account_kmem()

[Vladimir Davydov]

On 11/29/2013 01:45 PM, Michal Hocko wrote:
> 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?

Yes, I guess you're perfectly right and we don't need the ACTIVATED bit 
at all. I'll look at this deeper and send a patch removing it if it 
doesn't break something.

Thanks.

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