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

From: Mel Gorman
Date: Fri Nov 21 2014 - 04:35:19 EST

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@xxxxxxx> 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
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