Re: [PATCH v2] memcg: Prevent memcg caches to be both OFF_SLAB & OBJFREELIST_SLAB

[David Rientjes <rientjes@google.com>]

On Mon, 31 Oct 2016, Thomas Garnier wrote:

> While testing OBJFREELIST_SLAB integration with pagealloc, we found a
> bug where kmem_cache(sys) would be created with both CFLGS_OFF_SLAB &
> CFLGS_OBJFREELIST_SLAB.
> 
> The original kmem_cache is created early making OFF_SLAB not possible.
> When kmem_cache(sys) is created, OFF_SLAB is possible and if pagealloc
> is enabled it will try to enable it first under certain conditions.
> Given kmem_cache(sys) reuses the original flag, you can have both flags
> at the same time resulting in allocation failures and odd behaviors.
> 
> This fix discards allocator specific flags from memcg and ensure
> cache_create cannot be called with them.
> 
> Fixes: b03a017bebc4 ("mm/slab: introduce new slab management type, OBJFREELIST_SLAB")
> Signed-off-by: Thomas Garnier <thgarnie@google.com>
> Signed-off-by: Greg Thelen <gthelen@google.com>

Order of the signoffs is strange, should this have a

From: Greg Thelen <gthelen@google.com>

in the first line or is this your patch?

> ---
> Based on next-20161025
> ---
>  mm/slab.h        |  3 +++
>  mm/slab_common.c | 10 ++++++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> 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?

>  	err = -ENOMEM;
>  	s = kmem_cache_zalloc(kmem_cache, GFP_KERNEL);
>  	if (!s)
> @@ -533,8 +539,8 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
>  
>  	s = create_cache(cache_name, root_cache->object_size,
>  			 root_cache->size, root_cache->align,
> -			 root_cache->flags, root_cache->ctor,
> -			 memcg, root_cache);
> +			 root_cache->flags & SLAB_FLAGS_PERMITTED,
> +			 root_cache->ctor, memcg, root_cache);
>  	/*
>  	 * If we could not create a memcg cache, do not complain, because
>  	 * that's not critical at all as we can always proceed with the root

This introduces an inconsistency that isn't explained: why is SLAB_KASAN, 
the only reason why SLAB_FLAGS_PERMITTED needs to be defined, permitted 
for memcg_create_kmem_cache() but not kmem_cache_create()?  (If we need to 
keep SLAB_FLAGS_PERMITTED around, I think it needs a new name since its a 
restriction on the cache, not slab.)

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