[PATCH] memcg: make cache index determination more robust

[PATCH] memcg: make cache index determination more robust

[Glauber Costa]

I caught myself doing something like the following outside memcg core:

	memcg_id = -1;
	if (memcg && memcg_kmem_is_active(memcg))
		memcg_id = memcg_cache_id(memcg);

to be able to handle all possible memcgs in a sane manner. In particular, the
root cache will have kmemcg_id = -1 (just because we don't call memcg_kmem_init
to the root cache since it is not limitable). We have always coped with that by
making sure we sanitize which cache is passed to memcg_cache_id. Although this
example is given for root, what we really need to know is whether or not a
cache is kmem active.

But outside the memcg core testing for root, for instance, is not trivial since
we don't export mem_cgroup_is_root. I ended up realizing that this tests really
belong inside memcg_cache_id. This patch moves the tests inside memcg_cache_id
and make sure it always return a meaningful value.

Signed-off-by: Glauber Costa <glommer@openvz.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2e851f4..749f7a4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3081,7 +3081,9 @@ void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep)
  */
 int memcg_cache_id(struct mem_cgroup *memcg)
 {
-	return memcg ? memcg->kmemcg_id : -1;
+	if (!memcg || !memcg_kmem_is_active(memcg))
+		return -1;
+	return memcg->kmemcg_id;
 }
 
 /*
-- 
1.8.1.4

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Re: [PATCH] memcg: make cache index determination more robust

[Michal Hocko]

On Wed 12-06-13 16:43:28, Glauber Costa wrote:
> I caught myself doing something like the following outside memcg core:
> 
> 	memcg_id = -1;
> 	if (memcg && memcg_kmem_is_active(memcg))
> 		memcg_id = memcg_cache_id(memcg);
> 
> to be able to handle all possible memcgs in a sane manner. In particular, the
> root cache will have kmemcg_id = -1 (just because we don't call memcg_kmem_init
> to the root cache since it is not limitable). We have always coped with that by
> making sure we sanitize which cache is passed to memcg_cache_id. Although this
> example is given for root, what we really need to know is whether or not a
> cache is kmem active.
> 
> But outside the memcg core testing for root, for instance, is not trivial since
> we don't export mem_cgroup_is_root. I ended up realizing that this tests really
> belong inside memcg_cache_id. This patch moves the tests inside memcg_cache_id
> and make sure it always return a meaningful value.

This is quite a mess, to be honest. Some callers test/require
memcg_can_account_kmem others !p->is_root_cache. Can we have that
unified, please?

Also the return value of this function is used mostly as an index to
memcg_params->memcg_caches array so returning -1 sounds like a bad idea.
Few other cases use it as a real id. Maybe we need to split this up.

Pulling the check inside the function is OK but can we settle with a
common pattern here, pretty please?

> Signed-off-by: Glauber Costa <glommer@openvz.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.cz>
> Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  mm/memcontrol.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2e851f4..749f7a4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3081,7 +3081,9 @@ void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep)
>   */
>  int memcg_cache_id(struct mem_cgroup *memcg)
>  {
> -	return memcg ? memcg->kmemcg_id : -1;
> +	if (!memcg || !memcg_kmem_is_active(memcg))
> +		return -1;
> +	return memcg->kmemcg_id;
>  }
>  
>  /*
> -- 
> 1.8.1.4
> 

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH] memcg: make cache index determination more robust

[Glauber Costa]

On Thu, Jun 13, 2013 at 06:38:49PM +0200, Michal Hocko wrote:
> On Wed 12-06-13 16:43:28, Glauber Costa wrote:
> > I caught myself doing something like the following outside memcg core:
> > 
> > 	memcg_id = -1;
> > 	if (memcg && memcg_kmem_is_active(memcg))
> > 		memcg_id = memcg_cache_id(memcg);
> > 
> > to be able to handle all possible memcgs in a sane manner. In particular, the
> > root cache will have kmemcg_id = -1 (just because we don't call memcg_kmem_init
> > to the root cache since it is not limitable). We have always coped with that by
> > making sure we sanitize which cache is passed to memcg_cache_id. Although this
> > example is given for root, what we really need to know is whether or not a
> > cache is kmem active.
> > 
> > But outside the memcg core testing for root, for instance, is not trivial since
> > we don't export mem_cgroup_is_root. I ended up realizing that this tests really
> > belong inside memcg_cache_id. This patch moves the tests inside memcg_cache_id
> > and make sure it always return a meaningful value.
> 
> This is quite a mess, to be honest. Some callers test/require
> memcg_can_account_kmem others !p->is_root_cache. Can we have that
> unified, please?
> 
> Also the return value of this function is used mostly as an index to
> memcg_params->memcg_caches array so returning -1 sounds like a bad idea.
> Few other cases use it as a real id. Maybe we need to split this up.
> 
> Pulling the check inside the function is OK but can we settle with a
> common pattern here, pretty please?
> 

We have been through the array index discussion before. It is used as an array index
only in contexts where we are absolutely sure we are dealing with a memcg that is kmem
limited. Those are usually contexts in which in case it is not, we would have to BUG
anyway.

If you prefer, though, that we always BUG on id == -1 in those scenarios, for consistency,
this is understandable and I will prepare a patch for this.

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Re: [PATCH] memcg: make cache index determination more robust

[Glauber Costa]

On Thu, Jun 13, 2013 at 06:38:49PM +0200, Michal Hocko wrote:
> On Wed 12-06-13 16:43:28, Glauber Costa wrote:
> > I caught myself doing something like the following outside memcg core:
> > 
> > 	memcg_id = -1;
> > 	if (memcg && memcg_kmem_is_active(memcg))
> > 		memcg_id = memcg_cache_id(memcg);
> > 
> > to be able to handle all possible memcgs in a sane manner. In particular, the
> > root cache will have kmemcg_id = -1 (just because we don't call memcg_kmem_init
> > to the root cache since it is not limitable). We have always coped with that by
> > making sure we sanitize which cache is passed to memcg_cache_id. Although this
> > example is given for root, what we really need to know is whether or not a
> > cache is kmem active.
> > 
> > But outside the memcg core testing for root, for instance, is not trivial since
> > we don't export mem_cgroup_is_root. I ended up realizing that this tests really
> > belong inside memcg_cache_id. This patch moves the tests inside memcg_cache_id
> > and make sure it always return a meaningful value.
> 
> This is quite a mess, to be honest. Some callers test/require
> memcg_can_account_kmem others !p->is_root_cache. Can we have that
> unified, please?
> 
> Also the return value of this function is used mostly as an index to
> memcg_params->memcg_caches array so returning -1 sounds like a bad idea.
> Few other cases use it as a real id. Maybe we need to split this up.
> 
> Pulling the check inside the function is OK but can we settle with a
> common pattern here, pretty please?
> 
BTW: Since the test for memcg_can_account_kmem is a bit stronger than
memcg_kmem_is_active (the difference is that it tests the extra bit that we need
to coordinate the static branches), I will test for that, instead. Like this:

int memcg_cache_id(struct mem_cgroup *memcg)
{
        if (!memcg_can_account_kmem(memcg))                       
                return -1;
        return memcg->kmemcg_id;                                          
}

This will allow us to consolidate the tests around it a bit in my follow up patch.

> > Signed-off-by: Glauber Costa <glommer@openvz.org>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Michal Hocko <mhocko@suse.cz>
> > Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > ---
> >  mm/memcontrol.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 2e851f4..749f7a4 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3081,7 +3081,9 @@ void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep)
> >   */
> >  int memcg_cache_id(struct mem_cgroup *memcg)
> >  {
> > -	return memcg ? memcg->kmemcg_id : -1;
> > +	if (!memcg || !memcg_kmem_is_active(memcg))
> > +		return -1;
> > +	return memcg->kmemcg_id;
> >  }
> >  
> >  /*
> > -- 
> > 1.8.1.4
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs

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Re: [PATCH] memcg: make cache index determination more robust

[Michal Hocko]

On Fri 14-06-13 15:01:45, Glauber Costa wrote:
> On Thu, Jun 13, 2013 at 06:38:49PM +0200, Michal Hocko wrote:
> > On Wed 12-06-13 16:43:28, Glauber Costa wrote:
> > > I caught myself doing something like the following outside memcg core:
> > > 
> > > 	memcg_id = -1;
> > > 	if (memcg && memcg_kmem_is_active(memcg))
> > > 		memcg_id = memcg_cache_id(memcg);
> > > 
> > > to be able to handle all possible memcgs in a sane manner. In particular, the
> > > root cache will have kmemcg_id = -1 (just because we don't call memcg_kmem_init
> > > to the root cache since it is not limitable). We have always coped with that by
> > > making sure we sanitize which cache is passed to memcg_cache_id. Although this
> > > example is given for root, what we really need to know is whether or not a
> > > cache is kmem active.
> > > 
> > > But outside the memcg core testing for root, for instance, is not trivial since
> > > we don't export mem_cgroup_is_root. I ended up realizing that this tests really
> > > belong inside memcg_cache_id. This patch moves the tests inside memcg_cache_id
> > > and make sure it always return a meaningful value.
> > 
> > This is quite a mess, to be honest. Some callers test/require
> > memcg_can_account_kmem others !p->is_root_cache. Can we have that
> > unified, please?
> > 
> > Also the return value of this function is used mostly as an index to
> > memcg_params->memcg_caches array so returning -1 sounds like a bad idea.
> > Few other cases use it as a real id. Maybe we need to split this up.
> > 
> > Pulling the check inside the function is OK but can we settle with a
> > common pattern here, pretty please?
> > 
> 
> We have been through the array index discussion before. It is used as
> an array index only in contexts where we are absolutely sure we are
> dealing with a memcg that is kmem limited. Those are usually contexts
> in which in case it is not, we would have to BUG anyway.
>
> If you prefer, though, that we always BUG on id == -1 in those
> scenarios, for consistency, this is understandable and I will prepare
> a patch for this.

I think it would even make sense to have two things memcg_cache_id - the
same we have now + your condition enhancement - and memcg_cache_idx
which uses memcg_cache_id internally and BUG_ON(memcg_cache_id()==-1).
-- 
Michal Hocko
SUSE Labs

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Re: [PATCH] memcg: make cache index determination more robust

[Michal Hocko]

On Fri 14-06-13 15:24:00, Glauber Costa wrote:
> On Thu, Jun 13, 2013 at 06:38:49PM +0200, Michal Hocko wrote:
> > On Wed 12-06-13 16:43:28, Glauber Costa wrote:
> > > I caught myself doing something like the following outside memcg core:
> > > 
> > > 	memcg_id = -1;
> > > 	if (memcg && memcg_kmem_is_active(memcg))
> > > 		memcg_id = memcg_cache_id(memcg);
> > > 
> > > to be able to handle all possible memcgs in a sane manner. In particular, the
> > > root cache will have kmemcg_id = -1 (just because we don't call memcg_kmem_init
> > > to the root cache since it is not limitable). We have always coped with that by
> > > making sure we sanitize which cache is passed to memcg_cache_id. Although this
> > > example is given for root, what we really need to know is whether or not a
> > > cache is kmem active.
> > > 
> > > But outside the memcg core testing for root, for instance, is not trivial since
> > > we don't export mem_cgroup_is_root. I ended up realizing that this tests really
> > > belong inside memcg_cache_id. This patch moves the tests inside memcg_cache_id
> > > and make sure it always return a meaningful value.
> > 
> > This is quite a mess, to be honest. Some callers test/require
> > memcg_can_account_kmem others !p->is_root_cache. Can we have that
> > unified, please?
> > 
> > Also the return value of this function is used mostly as an index to
> > memcg_params->memcg_caches array so returning -1 sounds like a bad idea.
> > Few other cases use it as a real id. Maybe we need to split this up.
> > 
> > Pulling the check inside the function is OK but can we settle with a
> > common pattern here, pretty please?
> > 
> BTW: Since the test for memcg_can_account_kmem is a bit stronger than
> memcg_kmem_is_active (the difference is that it tests the extra bit that we need
> to coordinate the static branches), I will test for that, instead. Like this:
> 
> int memcg_cache_id(struct mem_cgroup *memcg)
> {
>         if (!memcg_can_account_kmem(memcg))                       
>                 return -1;
>         return memcg->kmemcg_id;                                          
> }

Makes sense. You also need to test memcg == NULL, right?

-- 
Michal Hocko
SUSE Labs

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