Date: Wed, 2 Nov 2016 17:46:07 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
To: Thomas Garnier <thgarnie@google.com>
cc: Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        Greg Thelen <gthelen@google.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v2] memcg: Prevent memcg caches to be both OFF_SLAB &
 OBJFREELIST_SLAB
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On Wed, 2 Nov 2016, Thomas Garnier wrote:

> >> diff --git a/mm/slab.h b/mm/slab.h
> >> index 9653f2e..58be647 100644
> >> --- a/mm/slab.h
> >> +++ b/mm/slab.h
> >> @@ -144,6 +144,9 @@ static inline unsigned long kmem_cache_flags(unsigned long object_size,
> >>
> >>  #define CACHE_CREATE_MASK (SLAB_CORE_FLAGS | SLAB_DEBUG_FLAGS | SLAB_CACHE_FLAGS)
> >>
> >> +/* Common allocator flags allowed for cache_create. */
> >> +#define SLAB_FLAGS_PERMITTED (CACHE_CREATE_MASK | SLAB_KASAN)
> >> +
> >>  int __kmem_cache_shutdown(struct kmem_cache *);
> >>  void __kmem_cache_release(struct kmem_cache *);
> >>  int __kmem_cache_shrink(struct kmem_cache *, bool);
> >> diff --git a/mm/slab_common.c b/mm/slab_common.c
> >> index 71f0b28..01d067c 100644
> >> --- a/mm/slab_common.c
> >> +++ b/mm/slab_common.c
> >> @@ -329,6 +329,12 @@ static struct kmem_cache *create_cache(const char *name,
> >>       struct kmem_cache *s;
> >>       int err;
> >>
> >> +     /* Do not allow allocator specific flags */
> >> +     if (flags & ~SLAB_FLAGS_PERMITTED) {
> >> +             err = -EINVAL;
> >> +             goto out;
> >> +     }
> >> +
> >
> > Why not just flags &= SLAB_FLAGS_PERMITTED if we're concerned about this
> > like kmem_cache_create does &= CACHE_CREATE_MASK?
> >
> 
> Christoph on the first version advised removing invalid flags on the
> caller and checking they are correct in kmem_cache_create. The memcg
> path putting the wrong flags is through create_cache but I still used
> this approach.
> 

I think this is a rather trivial point since it doesn't matter if we clear 
invalid flags on the caller or in the callee and obviously 
kmem_cache_create() does it in the callee.