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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc/32: disable KASAN with pages bigger than 16k
In-Reply-To: <afb1a3a9-8d7d-4c99-d42f-f6d78dcef0a5@csgroup.eu>
References: <7195fcde7314ccbf7a081b356084a69d421b10d4.1590660977.git.christophe.leroy@csgroup.eu> <afb1a3a9-8d7d-4c99-d42f-f6d78dcef0a5@csgroup.eu>
Date:   Tue, 02 Jun 2020 20:25:48 +1000
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Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> Le 28/05/2020 =C3=A0 12:17, Christophe Leroy a =C3=A9crit=C2=A0:
>> Mapping of early shadow area is implemented by using a single static
>> page table having all entries pointing to the same early shadow page.
>> The shadow area must therefore occupy full PGD entries.
>>=20
>> The shadow area has a size of 128Mbytes starting at 0xf8000000.
>> With 4k pages, a PGD entry is 4Mbytes
>> With 16k pages, a PGD entry is 64Mbytes
>> With 64k pages, a PGD entry is 256Mbytes which is too big.
>
> That's for 32k pages that a PGD is 256Mbytes.
>
> With 64k pages, a PGD entry is 1Gbytes which is too big.
>
> Michael, can you fix the commit log ?

Yes.

  powerpc/32: Disable KASAN with pages bigger than 16k
=20=20
  Mapping of early shadow area is implemented by using a single static
  page table having all entries pointing to the same early shadow page.
  The shadow area must therefore occupy full PGD entries.
=20=20
  The shadow area has a size of 128MB starting at 0xf8000000.
  With 4k pages, a PGD entry is 4MB
  With 16k pages, a PGD entry is 64MB
  With 64k pages, a PGD entry is 1GB which is too big.
=20=20
  Until we rework the early shadow mapping, disable KASAN when the page
  size is too big.

  Fixes: 2edb16efc899 ("powerpc/32: Add KASAN support")
  Cc: stable@vger.kernel.org # v5.2+
  Reported-by: kbuild test robot <lkp@intel.com>
  Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
  Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
  Link: https://lore.kernel.org/r/7195fcde7314ccbf7a081b356084a69d421b10d4.=
1590660977.git.christophe.leroy@csgroup.eu


cheers