Re: [RFC PATCH V2 2/9] mm: generalize avoid fault-inject on bootstrap kmem_cache

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Tue, Dec 08, 2015 at 05:18:32PM +0100, Jesper Dangaard Brouer wrote:
> Move slab_should_failslab() check from SLAB allocator to generic
> slab_pre_alloc_hook().  The check guards against slab alloc
> fault-injects failures for the bootstrap slab that is used for
> allocating "kmem_cache" objects to the allocator itself.
> 
> I'm not really happy with this code...
> ---
>  mm/failslab.c |    2 ++
>  mm/slab.c     |    8 --------
>  mm/slab.h     |   23 ++++++++++++++++++++++-
>  3 files changed, 24 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/failslab.c b/mm/failslab.c
> index 79171b4a5826..a2ad28ba696c 100644
> --- a/mm/failslab.c
> +++ b/mm/failslab.c
> @@ -13,6 +13,8 @@ static struct {
>  
>  bool should_failslab(size_t size, gfp_t gfpflags, unsigned long cache_flags)
>  {
> +	// Should we place bootstrap kmem_cache check here???
> +
>  	if (gfpflags & __GFP_NOFAIL)
>  		return false;
>  
> diff --git a/mm/slab.c b/mm/slab.c
> index 4765c97ce690..4684c2496982 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -2917,14 +2917,6 @@ static void *cache_alloc_debugcheck_after(struct kmem_cache *cachep,
>  #define cache_alloc_debugcheck_after(a,b,objp,d) (objp)
>  #endif
>  
> -static bool slab_should_failslab(struct kmem_cache *cachep, gfp_t flags)
> -{
> -	if (unlikely(cachep == kmem_cache))
> -		return false;
> -
> -	return should_failslab(cachep->object_size, flags, cachep->flags);
> -}
> -
>  static inline void *____cache_alloc(struct kmem_cache *cachep, gfp_t flags)
>  {
>  	void *objp;
> diff --git a/mm/slab.h b/mm/slab.h
> index 588bc5281fc8..4e7b0e62f3f4 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -360,6 +360,27 @@ static inline size_t slab_ksize(const struct kmem_cache *s)
>  }
>  #endif
>  
> +/* FIXME: This construct sucks, because this compare+branch needs to
> + * get removed by compiler then !CONFIG_FAILSLAB (maybe compiler is
> + * smart enough to realize only "false" can be generated).
> + *
> + * Comments please: Pulling out CONFIG_FAILSLAB here looks ugly...
> + *  should we instead change API of should_failslab() ??
> + *
> + * Next question: is the bootstrap cache check okay to add for all
> + * allocators? (this would be the easiest, else need more ugly ifdef's)
> + */
> +static inline bool slab_should_failslab(struct kmem_cache *cachep, gfp_t flags)
> +{
> +	/* No fault-injection for bootstrap cache */
> +#ifdef CONFIG_FAILSLAB
> +	if (unlikely(cachep == kmem_cache))
> +		return false;
> +#endif
> +
> +	return should_failslab(cachep->object_size, flags, cachep->flags);
> +}
> +
>  static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
>  						     gfp_t flags)
>  {
> @@ -367,7 +388,7 @@ static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
>  	lockdep_trace_alloc(flags);
>  	might_sleep_if(gfpflags_allow_blocking(flags));
>  
> -	if (should_failslab(s->object_size, flags, s->flags))
> +	if (slab_should_failslab(s, flags))
>  		return NULL;

It'd be better to remove slab_should_failslab() and insert following code
snippet here.

if (IS_ENABLED(CONFIG_FAILSLAB) &&
        cachep != kmem_cache &&
        should_failslab())
        return NULL;

Thanks.

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