linux-kernel - Re: [RFC v4 09/17] powerpc: call the hash functions with the correct pkey value

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Date:   Wed, 28 Jun 2017 00:12:28 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
        khandual@...ux.vnet.ibm.com, bsingharora@...il.com,
        dave.hansen@...el.com, hbabu@...ibm.com, arnd@...db.de,
        akpm@...ux-foundation.org, corbet@....net, mingo@...hat.com
Subject: Re: [RFC v4 09/17] powerpc: call the hash functions with the correct
 pkey value

On Tue, Jun 27, 2017 at 08:54:07PM +0530, Aneesh Kumar K.V wrote:
> 
> 
> On Tuesday 27 June 2017 03:41 PM, Ram Pai wrote:
> >Pass the correct protection key value to the hash functions on
> >page fault.
> >
> >Signed-off-by: Ram Pai <linuxram@...ibm.com>
> >---
> >  arch/powerpc/include/asm/pkeys.h | 11 +++++++++++
> >  arch/powerpc/mm/hash_utils_64.c  |  4 ++++
> >  arch/powerpc/mm/mem.c            |  6 ++++++
> >  3 files changed, 21 insertions(+)
> >
> >diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
> >index ef1c601..1370b3f 100644
> >--- a/arch/powerpc/include/asm/pkeys.h
> >+++ b/arch/powerpc/include/asm/pkeys.h
> >@@ -74,6 +74,17 @@ static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
> >  }
> >
> >  /*
> >+ * return the protection key of the vma corresponding to the
> >+ * given effective address @ea.
> >+ */
> >+static inline int mm_pkey(struct mm_struct *mm, unsigned long ea)
> >+{
> >+	struct vm_area_struct *vma = find_vma(mm, ea);
> >+	int pkey = vma ? vma_pkey(vma) : 0;
> >+	return pkey;
> >+}
> >+
> >+/*
> >
> 
> That is not going to work in hash fault path right ? We can't do a
> find_vma there without holding the mmap_sem

There is a fundamental problem with this new design. Looks like we can't
hold a lock in that path, without badly hurting the performance.

I am moving back to the old design. Cant by-pass the pte. The
keys will be programmed into the pte which will than be used
to program the hpte.

RP