linux-kernel - Re: [PATCH 2/3] x86: Define _PAGE_NUMA with unused physical address bits PMD and PTE levels

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Message-ID: <CAKbGBLhsWKVYnBqR0ZJ2kfaF_h=XAYkjq=v3RLoRBDkF_w=6ag@mail.gmail.com>
Date:	Mon, 7 Apr 2014 21:04:53 -0700
From:	Steven Noonan <steven@...inklabs.net>
To:	Mel Gorman <mgorman@...e.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Linux-MM <linux-mm@...ck.org>, Linux-X86 <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH 2/3] x86: Define _PAGE_NUMA with unused physical address
 bits PMD and PTE levels

On Mon, Apr 7, 2014 at 2:25 PM, Mel Gorman <mgorman@...e.de> wrote:
> On Mon, Apr 07, 2014 at 12:42:40PM -0700, H. Peter Anvin wrote:
>> On 04/07/2014 12:36 PM, Cyrill Gorcunov wrote:
>> > On Mon, Apr 07, 2014 at 12:27:10PM -0700, H. Peter Anvin wrote:
>> >> On 04/07/2014 11:28 AM, Mel Gorman wrote:
>> >>>
>> >>> I had considered the soft-dirty tracking usage of the same bit. I thought I'd
>> >>> be able to swizzle around it or a further worst case of having soft-dirty and
>> >>> automatic NUMA balancing mutually exclusive. Unfortunately upon examination
>> >>> it's not obvious how to have both of them share a bit and I suspect any
>> >>> attempt to will break CRIU.  In my current tree, NUMA_BALANCING cannot be
>> >>> set if MEM_SOFT_DIRTY which is not particularly satisfactory. Next on the
>> >>> list is examining if _PAGE_BIT_IOMAP can be used.
>> >>
>> >> Didn't we smoke the last user of _PAGE_BIT_IOMAP?
>> >
>> > Seems so, at least for non-kernel pages (not considering this bit references in
>> > xen code, which i simply don't know but i guess it's used for kernel pages only).
>> >
>>
>> David Vrabel has a patchset which I presumed would be pulled through the
>> Xen tree this merge window:
>>
>> [PATCHv5 0/8] x86/xen: fixes for mapping high MMIO regions (and remove
>> _PAGE_IOMAP)
>>
>> That frees up this bit.
>>
>
> Thanks, I was not aware of that patch.  Based on it, I intend to force
> automatic NUMA balancing to depend on !XEN and see what the reaction is. If
> support for Xen is really required then it potentially be re-enabled if/when
> that series is merged assuming they do not need the bit for something else.
>

Amazon EC2 does have large memory instance types with NUMA exposed to
the guest (e.g. c3.8xlarge, i2.8xlarge, etc), so it'd be preferable
(to me anyway) if we didn't require !XEN.
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