linux-kernel - Re: [PATCH] mm: compaction: Abort compaction if too many pages are isolated and caller is asynchronous

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Date:	Wed, 1 Jun 2011 22:40:18 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Minchan Kim <minchan.kim@...il.com>, akpm@...ux-foundation.org,
	Ury Stankevich <urykhy@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, stable@...nel.org
Subject: Re: [PATCH] mm: compaction: Abort compaction if too many pages are
 isolated and caller is asynchronous

On Wed, Jun 01, 2011 at 09:15:29PM +0200, Andrea Arcangeli wrote:
> On Wed, Jun 01, 2011 at 06:58:09PM +0100, Mel Gorman wrote:
> > Umm, HIGHMEM4G implies a two-level pagetable layout so where are
> > things like _PAGE_BIT_SPLITTING being set when THP is enabled?
> 
> They should be set on the pgd, pud_offset/pgd_offset will just bypass.
> The splitting bit shouldn't be special about it, the present bit
> should work the same.

This comment is misleading at best then.

#define _PAGE_BIT_SPLITTING     _PAGE_BIT_UNUSED1 /* only valid on a PSE pmd */

At the PGD level, it can have PSE set obviously but it's not a
PMD. I confess I haven't checked the manual to see if it's safe to
use _PAGE_BIT_UNUSED1 like this so am taking your word for it. I
found that the bug is far harder to reproduce with 3 pagetable levels
than with 2 but that is just timing. So far it has proven impossible
on x86-64 at least within 27 hours so that has me looking at how
pagetable management between x86 and x86-64 differ.

Barriers are a big different between how 32-bit !SMP and X86-64 but
don't know yet which one is relevant or if this is even the right
direction.

-- 
Mel Gorman
SUSE Labs
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