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Subject: Re: [Bug 202149] New: NULL Pointer Dereference in __split_huge_pmd on
 PPC64LE
From: Matt Corallo <kernel@bluematt.me>
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References: <bug-202149-27@https.bugzilla.kernel.org/>
 <20190104170459.c8c7fa57ba9bc8a69dee5666@linux-foundation.org>
 <87ef9nk4cj.fsf@linux.ibm.com>
 <ed4bea40-cf9e-89a1-f99a-3dbd6249847f@bluematt.me>
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 <A61367CF-277E-4E74-8A9D-C94C5E53817B@bluematt.me>
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To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
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Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Hey, sorry for the delay on this. I had some apparently-unrelated hangs that=
 I believe were due to mpt3sas instability, and at the risk of speaking too s=
oon for a bug I couldn't reliably reproduce, this patch appears to have reso=
lved it, thanks!

> On Jan 21, 2019, at 07:35, Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> w=
rote:
>=20
>=20
> Can you test this patch?
>=20
> =46rom e511e79af9a314854848ea8fda9dfa6d7e07c5e4 Mon Sep 17 00:00:00 2001
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> Date: Mon, 21 Jan 2019 16:43:17 +0530
> Subject: [PATCH] arch/powerpc/radix: Fix kernel crash with mremap
>=20
> With support for split pmd lock, we use pmd page pmd_huge_pte pointer to s=
tore
> the deposited page table. In those config when we move page tables we need=
 to
> make sure we move the depoisted page table to the right pmd page. Otherwis=
e this
> can result in crash when we withdraw of deposited page table because we ca=
n find
> the pmd_huge_pte NULL.
>=20
> c0000000004a1230 __split_huge_pmd+0x1070/0x1940
> c0000000004a0ff4 __split_huge_pmd+0xe34/0x1940 (unreliable)
> c0000000004a4000 vma_adjust_trans_huge+0x110/0x1c0
> c00000000042fe04 __vma_adjust+0x2b4/0x9b0
> c0000000004316e8 __split_vma+0x1b8/0x280
> c00000000043192c __do_munmap+0x13c/0x550
> c000000000439390 sys_mremap+0x220/0x7e0
> c00000000000b488 system_call+0x5c/0x70
>=20
> Fixes: 675d995297d4 ("powerpc/book3s64: Enable split pmd ptlock.")
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
> arch/powerpc/include/asm/book3s/64/pgtable.h | 2 --
> 1 file changed, 2 deletions(-)
>=20
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/i=
nclude/asm/book3s/64/pgtable.h
> index 92eaea164700..86e62384256d 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -1262,8 +1262,6 @@ static inline int pmd_move_must_withdraw(struct spin=
lock *new_pmd_ptl,
>                     struct spinlock *old_pmd_ptl,
>                     struct vm_area_struct *vma)
> {
> -    if (radix_enabled())
> -        return false;
>    /*
>     * Archs like ppc64 use pgtable to store per pmd
>     * specific information. So when we switch the pmd,
> --=20
> 2.20.1
>=20