linux-kernel - Re: [RFC PATCH 0/5] Use an alternative to _PAGE_PROTNONE for _PAGE

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Message-ID: <20140409062103.GA7294@gmail.com>
Date:	Wed, 9 Apr 2014 08:21:03 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mel Gorman <mgorman@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Linux-X86 <x86@...nel.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Steven Noonan <steven@...inklabs.net>,
	Rik van Riel <riel@...hat.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/5] Use an alternative to _PAGE_PROTNONE for
 _PAGE_NUMA v2


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Apr 08, 2014 at 05:46:52PM +0100, Mel Gorman wrote:
> > Someone will ask why automatic NUMA balancing hints do not use "real"
> > PROT_NONE but as it would need VMA information to do that on all
> > architectures it would mean that VMA-fixups would be required when marking
> > PTEs for NUMA hinting faults so would be expensive.
> 
> Like this:
> 
>   https://lkml.org/lkml/2012/11/13/431
> 
> That used the generic PROT_NONE infrastructure and compared, on fault,
> the page protection bits against the vma->vm_page_prot bits?
> 
> So the objection to that approach was the vma-> dereference in
> pte_numa() ?

I think the real underlying objection was that PTE_NUMA was the last 
leftover from AutoNUMA, and removing it would have made it not a 
'compromise' patch set between 'AutoNUMA' and 'sched/numa', but would 
have made the sched/numa approach 'win' by and large.

The whole 'losing face' annoyance that plagues all of us (me 
included).

I didn't feel it was important to the general logic of adding access 
pattern aware NUMA placement logic to the scheduler, and I obviously 
could not ignore the NAKs from various mm folks insisting on PTE_NUMA, 
so I conceded that point and Mel built on that approach as well.

Nice it's being cleaned up, and I'm pretty happy about how NUMA 
balancing ended up looking like.

Thanks,

	Ingo
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