linux-kernel - Re: [RFC v5 00/38] powerpc: Memory Protection Keys

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Date:   Mon, 10 Jul 2017 10:15:36 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Anshuman Khandual <khandual@...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,
        aneesh.kumar@...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 v5 00/38] powerpc: Memory Protection Keys

On Sun, Jul 09, 2017 at 11:05:44PM -0700, Ram Pai wrote:
> On Mon, Jul 10, 2017 at 11:13:23AM +0530, Anshuman Khandual wrote:
> > On 07/06/2017 02:51 AM, Ram Pai wrote:
.....
> 
> > do you have data points to show the difference in
> > performance between this version and the last one where
> > we skipped the bits from PTE and directly programmed the
> > HPTE entries looking into VMA bits.
> 
> No. I dont. I am hoping you can help me out with this.
Anshuman,
	The last version where we skipped the PTE bits is guaranteed
	to be bad/horrible. For one it has a bug, since it accesses
	the vma without a lock. And even if we did take a lock, it
	will slow down the page-hash path un-acceptably. So there is
	no point measuring the performance of that design.

	I think the number we want to measure is -- the performance 
	with the current design and comparing that to the performance
	without memkey feature. We want to find if there is
	any degradation by adding this feature.

RP