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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Thu, 18 Feb 2021 21:24:49 +0100
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Subject: Re: [PATCH RESEND] mm, kasan: don't poison boot memory
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        Peter Collingbourne <pcc@google.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Branislav Rankov <Branislav.Rankov@arm.com>,
        Kevin Brodsky <kevin.brodsky@arm.com>,
        Christoph Hellwig <hch@infradead.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        David Hildenbrand <david@redhat.com>,
        George Kennedy <george.kennedy@oracle.com>,
        Konrad Rzeszutek Wilk <konrad@darnok.org>
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On Thu, Feb 18, 2021 at 11:46 AM Catalin Marinas
<catalin.marinas@arm.com> wrote:
>
> The approach looks fine to me. If you don't like the trade-off, I think
> you could still leave the kasan poisoning in if CONFIG_DEBUG_KERNEL.

This won't work, Android enables CONFIG_DEBUG_KERNEL in GKI as it turns out :)

> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>
> Just curious, have you noticed any issue booting a KASAN_SW_TAGS-enabled
> kernel on a system with sufficiently large RAM? Is the boot slow-down
> significant?

When booting KASAN_SW_TAGS in QEMU with 40G there's a noticeable
start-up delay compared to 2G, but it doesn't seem to be caused by
this memblock->page_alloc poisoning, as removing it makes no
noticeable difference.

I also don't see a noticeable "hang" when booting KASAN_SW_TAGS in
FVP, compared to the one I see with KASAN_HW_TAGS. But I do see a
"hang" in QEMU when going from 2G to 40G with KASAN_HW_TAGS.

It seems that doing STG is much more expensive than writing to the
shadow memory.

> For MTE, we could look at optimising the poisoning code for page size to
> use STGM or DC GZVA but I don't think we can make it unnoticeable for
> large systems (especially with DC GZVA, that's like zeroing the whole
> RAM at boot).

https://bugzilla.kernel.org/show_bug.cgi?id=211817