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: <20140407212535.GJ7292@suse.de>
Date:	Mon, 7 Apr 2014 22:25:35 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Noonan <steven@...inklabs.net>,
	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 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.

-- 
Mel Gorman
SUSE Labs
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