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Date: Wed, 9 Apr 2014 17:12:24 -0700
Message-ID: <CA+55aFwMVjsYpT+c0GukgU18_YEKKWtXVpOk0ePRAtCDU71jqA@mail.gmail.com>
Subject: Re: [RFC PATCH 0/5] Use an alternative to _PAGE_PROTNONE for
 _PAGE_NUMA v2
From: Linus Torvalds <torvalds@linux-foundation.org>
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Sender: owner-linux-mm@kvack.org
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To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Mel Gorman <mgorman@suse.de>, Linux-X86 <x86@kernel.org>, Cyrill Gorcunov <gorcunov@gmail.com>, Steven Noonan <steven@uplinklabs.net>, Rik van Riel <riel@redhat.com>, David Vrabel <david.vrabel@citrix.com>, Andrew Morton <akpm@linux-foundation.org>, Andrea Arcangeli <aarcange@redhat.com>, Dave Hansen <dave.hansen@intel.com>, Srikar Dronamraju <srikar@linux.vnet.ibm.com>, Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

On Wed, Apr 9, 2014 at 4:34 PM, H. Peter Anvin <hpa@zytor.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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