linux-kernel - Re: [PATCH] kasan, slub: fix handling of kasan_slab

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Date:   Wed, 7 Mar 2018 15:43:51 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Kostya Serebryany <kcc@...gle.com>
Subject: Re: [PATCH] kasan, slub: fix handling of kasan_slab_free hook



On 03/06/2018 08:42 PM, Andrey Konovalov wrote:

>>> -     if (s->flags & SLAB_KASAN && !(s->flags & SLAB_TYPESAFE_BY_RCU))
>>> -             return;
>>> -     do_slab_free(s, page, head, tail, cnt, addr);
>>> +     slab_free_freelist_hook(s, &head, &tail);
>>> +     if (head != NULL)
>>
>> That's an additional branch in non-debug fast-path. Find a way to avoid this.
> 
> Hm, there supposed to be a branch here. We either have objects that we
> need to free, or we don't, and we need to do different things in those
> cases. Previously this was done with a hardcoded "if (s->flags &
> SLAB_KASAN && ..." statement, not it's a different "if (head !=
> NULL)".
> 

They are different. "if (s->flags & SLAB_KASAN && ..." can be optimized away by compiler when CONFIG_KASAN=n,
"if (head != NULL)" - can not.