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

[Vlastimil Babka <vbabka@suse.cz>]

On 6/25/24 7:12 PM, Alexei Starovoitov wrote:
> On Tue, Jun 25, 2024 at 7:24 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> 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
> 
> To prevent such compiler transformations we typically use
> __used noinline
> 
> We didn't have a need for __noclone yet. If __used is enough I'd stick to that.

__used made no difference here (gcc 13.3), __noclone did