linux-kernel - Re: 2.6.38 page

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Message-ID: <20110415091138.GB30898@csn.ul.ie>
Date:	Fri, 15 Apr 2011 10:11:38 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	raz ben yehuda <raziebe@...il.com>,
	lkml <linux-kernel@...r.kernel.org>, riel@...hat.com,
	kosaki.motohiro@...fujitsu.com, akpm@...ux-foundation.org
Subject: Re: 2.6.38 page_test regression

On Fri, Apr 15, 2011 at 01:44:44AM +0200, Andrea Arcangeli wrote:
> On Fri, Apr 15, 2011 at 01:32:26AM +0200, Andrea Arcangeli wrote:
> > On Thu, Apr 14, 2011 at 10:53:27PM +0100, Mel Gorman wrote:
> > > On Thu, Apr 14, 2011 at 11:07:23PM +0300, raz ben yehuda wrote:
> > > > bah. Mel is correct. I did mean page_test  ( in my defense it is in the
> > > > msg ).
> > > > Here some more information:
> > > > 1. I manage to lower the regression to 2 sha1's:
> > > >     	32dba98e085f8b2b4345887df9abf5e0e93bfc12 to
> > > > 71e3aac0724ffe8918992d76acfe3aad7d8724a5. 
> > > > 	though I had to remark wait_split_huge_page for the sake of
> > > > compilation. up to 32dba98e085f8b2b4345887df9abf5e0e93bfc12 there is no
> > > > regression.
> > > > 
> > > > 2. I booted 2.6.37-rc5 you gave me. same regression is there. 
> > > 
> > > Extremely long shot - try this patch.
> > > 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index c50a195..a39baaf 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -3317,7 +3317,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > >  	 * run pte_offset_map on the pmd, if an huge pmd could
> > >  	 * materialize from under us from a different thread.
> > >  	 */
> > > -	if (unlikely(__pte_alloc(mm, vma, pmd, address)))
> > > +	if (unlikely(!pmd_present(*(pmd))) && __pte_alloc(mm, vma, pmd, address))
> 
> Actually reading this again this should be pmd_none.

You're right.

> This is one
> change I had to make across the board because for an tiny amount of
> time during the pmd teardown I have to mark an pmd_trans_splitting pte
> not present in order to flush it away from the 2m tlb before the 4k
> tlb can be loaded (for errata), but when it's being splitted it's
> definitely not null. Now it's not buggy because then __pte_alloc would
> then check it against pmd_none, but it's not worth calling into
> __pte_alloc if it's not pmd_none. It just makes it confusing when
> everything has been updated to check pmd_none.
> 
> Just a nitpick of course (not even a bug).
> 

A fairly important nitpick. Otherwise the inconsistency is confusing.

-- 
Mel Gorman
SUSE Labs
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