Re: [PATCH] mm: don't call should_failslab() for !CONFIG_FAILSLAB

[Jens Axboe <axboe@kernel.dk>]

On 10/7/21 9:32 AM, Vlastimil Babka wrote:
> On 10/5/21 17:31, Jens Axboe wrote:
>> Allocations can be a very hot path, and this out-of-line function
>> call is noticeable.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> It used to be inline b4 (hi, Konstantin!) and then was converted to be like
> this intentionally :/
> 
> See 4f6923fbb352 ("mm: make should_failslab always available for fault
> injection")
> 
> And now also kernel/bpf/verifier.c contains:
> BTF_ID(func, should_failslab)
> 
> I think either your or Andrew's version will break this BTF_ID thing, at the
> very least.
> 
> But I do strongly agree that putting unconditionally a non-inline call into
> slab allocator fastpath sucks. Can we make it so that bpf can only do these
> overrides when CONFIG_FAILSLAB is enabled?
> I don't know, perhaps putting this BTF_ID() in #ifdef as well, or providing
> a dummy that is always available (so that nothing breaks), but doesn't
> actually affect slab_pre_alloc_hook() unless CONFIG_FAILSLAB has been enabled?

That seems to be the right approach, limiting it on it actually being enabled
and a function call.

-- 
Jens Axboe