linux-kernel - Re: [patch 1/3 take2] smaps: extract pte walker from smaps code

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Date:	Thu, 8 Feb 2007 20:00:39 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Paul Mundt <lethal@...ux-sh.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...l.org>, Hugh Dickins <hugh@...itas.com>,
	Christoph Lameter <clameter@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/3 take2] smaps: extract pte walker from smaps code

On Wed, Feb 07, 2007 at 03:29:34PM +0900, Paul Mundt wrote:
> On Tue, Feb 06, 2007 at 10:15:47PM -0800, David Rientjes wrote:
> > Extracts the page table entry walker from the smaps-specific code in
> > fs/proc/task_mmu.c.  This will be used later for clearing the reference
> > bits on pages to measure the number of pages accessed over a time period
> > through /proc/pid/smaps.
> > 
> I like the general idea of this patch set, however..
> 
> > Since the PTE walker is now extracted from the smaps code,
> > smaps_pte_func() is invoked for each PTE in the VMA.  Its behavior is
> > identical to the existing implementation, except it is slightly slower
> > because each PTE now invokes a function call.
> > 
> Perhaps this is something that needs to be looked at more closely and
> made more generic? There are many ranged page table walkers that aren't
> so performance critical that the function call cost would cause too much
> pain. ioremap_page_range() comes to mind, and there's bound to be others.
> This would also help people to get the pte map/unmap right, which seems
> to pop up from time to time as well..

I've been looking at a similar refactoring of other code and I think
the way to go is a callback per block-of-PTEs with start and end
pointers. That gets rid of most of the call indirection overhead.

-- 
Mathematics is the supreme nostalgia of our time.
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