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Date:   Tue, 5 Oct 2021 15:20:18 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: don't call should_failslab() for !CONFIG_FAILSLAB

On 10/5/21 3:18 PM, Andrew Morton wrote:
> On Tue, 5 Oct 2021 09:31:43 -0600 Jens Axboe <axboe@...nel.dk> wrote:
> 
>> Allocations can be a very hot path, and this out-of-line function
>> call is noticeable.
>>
>> --- a/include/linux/fault-inject.h
>> +++ b/include/linux/fault-inject.h
>> @@ -64,8 +64,8 @@ static inline struct dentry *fault_create_debugfs_attr(const char *name,
>>  
>>  struct kmem_cache;
>>  
>> -int should_failslab(struct kmem_cache *s, gfp_t gfpflags);
>>  #ifdef CONFIG_FAILSLAB
>> +int should_failslab(struct kmem_cache *s, gfp_t gfpflags);
>>  extern bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags);
>>  #else
>>  static inline bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags)
>> diff --git a/mm/slab.h b/mm/slab.h
>> index 58c01a34e5b8..92fd6fe01877 100644
>> --- a/mm/slab.h
>> +++ b/mm/slab.h
>> @@ -491,8 +491,10 @@ static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
>>  
>>  	might_alloc(flags);
>>  
>> +#ifdef CONFIG_FAILSLAB
>>  	if (should_failslab(s, flags))
>>  		return NULL;
>> +#endif
> 
> Can we avoid the ifdefs here?
> 
>>  
>>  	if (!memcg_slab_pre_alloc_hook(s, objcgp, size, flags))
>>  		return NULL;
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index ec2bb0beed75..c21bd447f237 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -1323,6 +1323,7 @@ EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc_node);
>>  EXPORT_TRACEPOINT_SYMBOL(kfree);
>>  EXPORT_TRACEPOINT_SYMBOL(kmem_cache_free);
>>  
>> +#ifdef CONFIG_FAILSLAB
>>  int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
>>  {
>>  	if (__should_failslab(s, gfpflags))
>> @@ -1330,3 +1331,4 @@ int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
>>  	return 0;
>>  }
>>  ALLOW_ERROR_INJECTION(should_failslab, ERRNO);
>> +#endif
> 
> Like,
> 
> --- a/include/linux/fault-inject.h~mm-dont-call-should_failslab-for-config_failslab-fix
> +++ a/include/linux/fault-inject.h
> @@ -68,6 +68,10 @@ struct kmem_cache;
>  int should_failslab(struct kmem_cache *s, gfp_t gfpflags);
>  extern bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags);
>  #else
> +static inline int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
> +{
> +	return 0;
> +}
>  static inline bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags)
>  {
>  	return false;
> --- a/mm/slab.h~mm-dont-call-should_failslab-for-config_failslab-fix
> +++ a/mm/slab.h
> @@ -491,10 +491,8 @@ static inline struct kmem_cache *slab_pr
>  
>  	might_alloc(flags);
>  
> -#ifdef CONFIG_FAILSLAB
>  	if (should_failslab(s, flags))
>  		return NULL;
> -#endif
>  
>  	if (!memcg_slab_pre_alloc_hook(s, objcgp, size, flags))
>  		return NULL;
> _

Yep, that'll work!

-- 
Jens Axboe

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