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Date:   Tue, 26 May 2020 10:53:30 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Vlastimil Babka <vbabka@suse.cz>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, <linux-mm@kvack.org>,
        <kernel-team@fb.com>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 08/19] mm: memcg/slab: save obj_cgroup for non-root
 slab objects
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On Mon, May 25, 2020 at 05:07:22PM +0200, Vlastimil Babka wrote:
> On 4/22/20 10:46 PM, Roman Gushchin wrote:
> > Store the obj_cgroup pointer in the corresponding place of
> > page->obj_cgroups for each allocated non-root slab object.
> > Make sure that each allocated object holds a reference to obj_cgroup.
> > 
> > Objcg pointer is obtained from the memcg->objcg dereferencing
> > in memcg_kmem_get_cache() and passed from pre_alloc_hook to
> > post_alloc_hook. Then in case of successful allocation(s) it's
> > getting stored in the page->obj_cgroups vector.
> > 
> > The objcg obtaining part look a bit bulky now, but it will be simplified
> > by next commits in the series.
> > 
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> 
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> 
> Nit below:
> 
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 44def57f050e..525e09e05743 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> ...
> > @@ -636,8 +684,8 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s, gfp_t flags,
> >  					 s->flags, flags);
> >  	}
> >  
> > -	if (memcg_kmem_enabled())
> > -		memcg_kmem_put_cache(s);
> > +	if (!is_root_cache(s))
> > +		memcg_slab_post_alloc_hook(s, objcg, size, p);
> >  }
> >  
> >  #ifndef CONFIG_SLOB
> 
> Keep also the memcg_kmem_enabled() static key check, like elsewhere?
> 

Ok, will add, it can speed things up a little bit. My only concern is that
the code is not ready for memcg_kmem_enabled() turning negative after being positive.
But it's not a concern, right?

Actually, we can simplify memcg_kmem_enabled() mechanics and enable it
only once as soon as the first memcg is fully initialized. I don't think there
is any value in tracking the actual number of active memcgs.

Thanks!