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

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Date:	Tue, 8 Apr 2014 19:51:46 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Linux-X86 <x86@...nel.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>
Subject: Re: [RFC PATCH 0/5] Use an alternative to _PAGE_PROTNONE for
 _PAGE_NUMA v2

On Tue, Apr 08, 2014 at 10:01:39AM -0700, Linus Torvalds wrote:
> On Tue, Apr 8, 2014 at 9:46 AM, Mel Gorman <mgorman@...e.de> wrote:
> >
> > If you are ok with leaving _PAGE_NUMA as _PAGE_PROTNONE
> 
> NO I AM NOT!
> 
> Dammit, this feature is f*cking brain-damaged.
> 
> My complaint has been (and continues to be):
> 
>  - either it is 100% the same as PROTNONE, in which case thjat
> _PAGE_NUMA bit had better go away, and you just use the protnone
> helpers!
> 

In which case we'd still use VMAs to distinguish between PROTNONE faults
and NUMA hinting faults. We may still need some special casing. It's plan
b but not my preferred solution at this time.

>  - if it's not the same as PROTNONE, then it damn well needs a different bit.
> 

With this series applied _PAGE_NUMA != _PAGE_PROTNONE.

> You can't have it both ways. You guys tried. The Xen case shows that
> trying to distinguish the two DOES NOT WORK. But even apart from the
> Xen case, it was just a confusing hell.
> 

Which is why I responded with a series that used a different bit instead
of more discussions that would reach the same conclusion. 

> Like Yoda said: "Either they are the same or they are not. There is no 'try'".
> 
> So pick one solution. Don't try to pick the mixed-up half-way case
> that is a disaster and makes no sense.
> 

I picked a solution. The posted series uses a different bit.

-- 
Mel Gorman
SUSE Labs
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