Re: [PATCH v2 6/7] mm, slab: add static key for should_failslab()

[Vlastimil Babka <vbabka@suse.cz>]

On 6/20/24 12:49 AM, Vlastimil Babka wrote:
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3874,13 +3874,37 @@ static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
>  			0, sizeof(void *));
>  }
>  
> -noinline int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
> +#if defined(CONFIG_FUNCTION_ERROR_INJECTION) || defined(CONFIG_FAILSLAB)
> +DEFINE_STATIC_KEY_FALSE(should_failslab_active);
> +
> +#ifdef CONFIG_FUNCTION_ERROR_INJECTION
> +noinline
> +#else
> +static inline
> +#endif
> +int should_failslab(struct kmem_cache *s, gfp_t gfpflags)

Note that it has been found that (regardless of this series) gcc may clone
this to a should_failslab.constprop.0 in case the function is empty because
__should_failslab is compiled out (CONFIG_FAILSLAB=n). The "noinline"
doesn't help - the original function stays but only the clone is actually
being called, thus overriding the original function achieves nothing, see:
https://github.com/bpftrace/bpftrace/issues/3258

So we could use __noclone to prevent that, and I was thinking by adding
something this to error-injection.h:

#ifdef CONFIG_FUNCTION_ERROR_INJECTION
#define __error_injectable(alternative)		noinline __noclone
#else
#define __error_injectable(alternative)		alternative
#endif

and the usage here would be:

__error_injectable(static inline) int should_failslab(...)

Does that look acceptable, or is it too confusing that "static inline" is
specified there as the storage class to use when error injection is actually
disabled?

>  {
>  	if (__should_failslab(s, gfpflags))
>  		return -ENOMEM;
>  	return 0;
>  }
> -ALLOW_ERROR_INJECTION(should_failslab, ERRNO);
> +ALLOW_ERROR_INJECTION_KEY(should_failslab, ERRNO, &should_failslab_active);
> +
> +static __always_inline int should_failslab_wrapped(struct kmem_cache *s,
> +						   gfp_t gfp)
> +{
> +	if (static_branch_unlikely(&should_failslab_active))
> +		return should_failslab(s, gfp);
> +	else
> +		return 0;
> +}
> +#else
> +static __always_inline int should_failslab_wrapped(struct kmem_cache *s,
> +						   gfp_t gfp)
> +{
> +	return false;
> +}
> +#endif
>  
>  static __fastpath_inline
>  struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s, gfp_t flags)
> @@ -3889,7 +3913,7 @@ struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s, gfp_t flags)
>  
>  	might_alloc(flags);
>  
> -	if (unlikely(should_failslab(s, flags)))
> +	if (should_failslab_wrapped(s, flags))
>  		return NULL;
>  
>  	return s;
>