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

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Date:	Wed, 9 Apr 2014 17:12:24 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>, 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

On Wed, Apr 9, 2014 at 4:34 PM, H. Peter Anvin <hpa@...or.com> wrote:
>
> How painful would it be to get rid of _PAGE_NUMA entirely?  Page bits
> are a highly precious commodity and saving one would be valuable.

I don't think _PAGE_NUMA is a problem. It's only set when the page is
not present, so we have tons of bits then.

Now, that's still inconvenient for the 32-bit pte case, because we do
*not* have tons of bits for non-present cases since we need them for
the swap indexes.

This is different from _PAGE_SOFT_DIRTY, which we do need for both
present and swapped-out entries.

Or am I missing something?

            Linus
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