linux-kernel - Re: [PATCH 02/10] mm: Add p[te|md] protnone helpers for use by NUMA balancing

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Message-ID: <20141121093506.GV2725@suse.de>
Date:	Fri, 21 Nov 2014 09:35:06 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	LinuxPPC-dev <linuxppc-dev@...ts.ozlabs.org>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Hugh Dickins <hughd@...gle.com>, Dave Jones <davej@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 02/10] mm: Add p[te|md] protnone helpers for use by NUMA
 balancing

On Thu, Nov 20, 2014 at 11:54:06AM -0800, Linus Torvalds wrote:
> On Thu, Nov 20, 2014 at 2:19 AM, Mel Gorman <mgorman@...e.de> wrote:
> > This is a preparatory patch that introduces protnone helpers for automatic
> > NUMA balancing.
> 
> Oh, I hadn't noticed that you had renamed these things. It was
> probably already true in your V1 version.
> 
> I do *not* think that "pte_protnone_numa()" makes sense as a name. It
> only confuses people to think that there is still/again something
> NUMA-special about the PTE. The whole point of the protnone changes
> was to make it really very very clear that from a hardware standpoint,
> this is *exactly* about protnone, and nothing else.
> 
> The fact that we then use protnone PTE's for numa faults is a VM
> internal issue, it should *not* show up in the architecture page table
> helpers.
> 
> I'm not NAK'ing this name, but I really think it's a very important
> part of the whole patch series - to stop the stupid confusion about
> NUMA entries. As far as the page tables are concerned, this has
> absolutely _zero_ to do with NUMA.
> 
> We made that mistake once. We're fixing it. Let the naming *show* that
> it's fixed, and this is "pte_protnone()".
> 
> The places that use this for NUMA handling might have a comment or
> something. But they'll be in the VM where this matters, not in the
> architecture page table description files. The comment would be
> something like "if the vma is accessible, but the PTE is marked
> protnone, this is a autonuma entry".
> 

I feared that people would eventually make the mistake of thinking that
pte_protnone() would return true for PROT_NONE VMAs that do *not* have
the page table bit set. I'll use the old name as you suggest and expand
the comment. It'll be in v3.

-- 
Mel Gorman
SUSE Labs
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