linux-kernel - Re: [PATCH v3] mm: make minimum slab alignment a runtime property

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Date:   Sun, 24 Apr 2022 12:15:19 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Peter Collingbourne <pcc@...gle.com>
cc:     Andrey Konovalov <andreyknvl@...il.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        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>,
        vbabka@...e.cz, penberg@...nel.org, roman.gushchin@...ux.dev,
        iamjoonsoo.kim@....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 v3] mm: make minimum slab alignment a runtime property

On Fri, 22 Apr 2022, Peter Collingbourne wrote:

> When CONFIG_KASAN_HW_TAGS is enabled we currently increase the minimum
> slab alignment to 16. This happens even if MTE is not supported in
> hardware or disabled via kasan=off, which creates an unnecessary
> memory overhead in those cases. Eliminate this overhead by making
> the minimum slab alignment a runtime property and only aligning to
> 16 if KASAN is enabled at runtime.
> 
> On a DragonBoard 845c (non-MTE hardware) with a kernel built with
> CONFIG_KASAN_HW_TAGS, waiting for quiescence after a full Android
> boot I see the following Slab measurements in /proc/meminfo (median
> of 3 reboots):
> 
> Before: 169020 kB
> After:  167304 kB
> 
> Link: https://linux-review.googlesource.com/id/I752e725179b43b144153f4b6f584ceb646473ead
> Signed-off-by: Peter Collingbourne <pcc@...gle.com>
> Reviewed-by: Andrey Konovalov <andreyknvl@...il.com>

Acked-by: David Rientjes <rientjes@...gle.com>

The command line options are described by 
Documentation/dev-tools/kasan.rst but it doesn't look like a update is 
necessary.  I think the assumption today is that if we're using kasan=off 
then we aren't doing the alignment.

I do wonder why kasan=off is not at least mentioned in 
Documentation/admin-guide/kernel-parameters.txt and perhaps for all other 
kasan options point the reader to Documentation/dev-tools/kasan.rst.