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: <20140407191622.GA23983@moon>
Date:	Mon, 7 Apr 2014 23:16:22 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Anvin <hpa@...or.com>, 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 07:28:54PM +0100, Mel Gorman wrote:
> > > I didn't bother spelling it out in case I gave the impression that I was
> > > blaming Xen for the problem.  As the bit is now changes, does it help
> > > the Xen problem or cause another collision of some sort? There is no
> > > guarantee _PAGE_NUMA will remain as bit 62 but at worst it'll use bit 11
> > > and NUMA_BALANCING will depend in !KMEMCHECK.
> > 
> > Fwiw, we're using bit 11 for soft-dirty tracking, so i really hope worst case
> > never happen. (At the moment I'm trying to figure out if with this set
> > it would be possible to clean up ugly macros in pgoff_to_pte for 2 level pages).
> 
> 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.

Thanks for info, Mel! It seems indeed if no more space left on x86-64 (in
the very worst case which I still think won't happen anytime soon) we'll
have to make them mut. exclusive. But for now (with 62 bit used for numa)
they can live together, right?
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