linux-kernel - Re: [PATCH] mm: make minimum slab alignment a runtime property

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Date:   Fri, 22 Apr 2022 18:09:04 +0200
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     Peter Collingbourne <pcc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pekka Enberg <penberg@...nel.org>, cl@...ux.org,
        roman.gushchin@...ux.dev, Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] mm: make minimum slab alignment a runtime property

On Fri, Apr 22, 2022 at 2:39 PM Hyeonggon Yoo <42.hyeyoo@...il.com> wrote:
>
> > > kasan_hw_tags_enabled() is also false when kasan is just not initialized yet.
> > > What about writing a new helper something like kasan_is_disabled()
> > > instead?
> >
> > The decision of whether to enable KASAN is made early, before the slab
> > allocator is initialized (start_kernel -> smp_prepare_boot_cpu ->
> > kasan_init_hw_tags vs start_kernel -> mm_init -> kmem_cache_init). If
> > you think about it, this needs to be the case for KASAN to operate
> > correctly because it influences the behavior of the slab allocator via
> > the kasan_*poison* hooks. So I don't think we can end up calling this
> > function before then.
>
> Sounds not bad. I wanted to make sure the value of arch_slab_minaligned()
> is not changed during its execution.
>
> Just some part of me thought something like this would be more
> intuitive/robust.
>
> if (systems_supports_mte() && kasan_arg != KASAN_ARG_OFF)
>         return MTE_GRANULE_SIZE;
> else
>         return __alignof__(unsigned long long);

Hi Hyeonggon,

We could add and use kasan_hw_rags_requested(), which would return
(systems_supports_mte() && kasan_arg != KASAN_ARG_OFF).

However, I'm not sure we will get a fully static behavior:
systems_supports_mte() also only starts returning proper result at
some point during CPU bring-up if I'm not mistaken.

Thanks!