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Subject: Re: [PATCH] mm: don't call should_failslab() for !CONFIG_FAILSLAB
To:     Vlastimil Babka <vbabka@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Howard McLauchlan <hmclauchlan@fb.com>
Cc:     bpf@vger.kernel.org
References: <e01e5e40-692a-519c-4cba-e3331f173c82@kernel.dk>
 <2dfc6273-6cdd-f4f5-bed9-400873ac9152@suse.cz>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <082f30c7-9a7c-b2de-6d30-99fa38150d48@kernel.dk>
Date:   Thu, 7 Oct 2021 09:50:11 -0600
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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