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

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Date:	Tue, 08 Apr 2014 07:40:29 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mel Gorman <mgorman@...e.de>, Linux-X86 <x86@...nel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Noonan <steven@...inklabs.net>,
	Rik van Riel <riel@...hat.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.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>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [RFC PATCH 0/5] Use an alternative to _PAGE_PROTNONE for _PAGE_NUMA
 v2

On 04/08/2014 06:09 AM, Mel Gorman wrote:
> Using unused physical bits is something that will break eventually.
> 
> Changelog since V1
> o Reuse software-bits
> o Use paravirt ops when modifying PTEs in the NUMA helpers
> 
> Aliasing _PAGE_NUMA and _PAGE_PROTNONE had some convenient properties but
> it ultimately gave Xen a headache and pisses almost everybody off that
> looks closely at it. Two discussions on "why this makes sense" is one
> discussion too many so rather than having a third so here is this series.
> This series reuses the PTE bits that are available to the programmer.
> This adds some contraints on how and when automatic NUMA balancing can be
> enabled but it should go away again when Xen stops using _PAGE_IOMAP.
> 
> The series also converts the NUMA helpers to use paravirt-friendly operations
> but it needs a Tested-by from the Xen and powerpc people.
> 

It is proably simpler to just base this patchset on top of David
Vrabel's which actually *does* remove _PAGE_IOMAP.

David, is your patchset going to be pushed in this merge window as expected?

That being said, these bits are precious, and if this ends up being a
case where "only Xen needs another bit" once again then Xen should
expect to get kicked to the curb at a moment's notice.

	-hpa


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