linux-kernel - Re: [patch][v2] swap: virtual swap readahead

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Date:	Wed, 03 Jun 2009 10:51:36 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch][v2] swap: virtual swap readahead

Johannes Weiner wrote:
> On Wed, Jun 03, 2009 at 01:34:57AM +0200, Andi Kleen wrote:
>> On Wed, Jun 03, 2009 at 12:37:39AM +0200, Johannes Weiner wrote:

>>> +		pgd = pgd_offset(vma->vm_mm, pos);
>>> +		if (!pgd_present(*pgd))
>>> +			continue;
>>> +		pud = pud_offset(pgd, pos);
>>> +		if (!pud_present(*pud))
>>> +			continue;
>>> +		pmd = pmd_offset(pud, pos);
>>> +		if (!pmd_present(*pmd))
>>> +			continue;
>>> +		pte = pte_offset_map_lock(vma->vm_mm, pmd, pos, &ptl);
>> You could be more efficient here by using the standard mm/* nested loop
>> pattern that avoids relookup of everything in each iteration. I suppose
>> it would mainly make a difference with 32bit highpte where mapping a pte
>> can be somewhat costly. And you would take less locks this way.
> 
> I ran into weird problems here.  The above version is actually faster
> in the benchmarks than writing a nested level walker or using
> walk_page_range().  Still digging but it can take some time.  Busy
> week :(

I'm not too worried about not walking the page tables,
because swap is an extreme slow path anyway.

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