linux-kernel - Re: [patch -mm] i386: use pte_update_defer in ptep_test_and_clear

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Message-ID: <Pine.LNX.4.64.0704161217310.12685@chino.kir.corp.google.com>
Date:	Mon, 16 Apr 2007 12:20:09 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Hugh Dickins <hugh@...itas.com>
cc:	Zachary Amsden <zach@...are.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch -mm] i386: use pte_update_defer in ptep_test_and_clear_{dirty,young}

On Mon, 16 Apr 2007, Hugh Dickins wrote:

> > > Compromise patch below: would that be satisfactory to you, David?
> > 
> > I really like the patch, but for perhaps a slightly different reason: 
> > we're only flushing ranges that have been shown to need it.  We aren't 
> > completely flushing the entire mm which is likely to be excessive in 
> > situations where we're actually using /proc/pid/clear_refs in combination 
> > with /proc/pid/smaps for memory footprint approximation (i.e. it's on a 
> > fine granularity).
> 
> It would be more of a reason to like the patch, if more architectures
> actually implemented flush_tlb_range as anything different from
> flush_tlb_mm ;)  Sadly, few can do better than flush_tlb_mm: ia64
> is the exception I remember, and maybe a couple of others.  I put
> flush_tlb_range there merely because it seems more appropriate,
> but it's rather deceptive.
> 

Sure, but what I really like about the patch is that we're only flushing 
something if !flush_end in the first place.  So we can eliminate any TLB 
flushing if that VMA didn't need it; that's a change from the current 
behavior.  And since the most obvious use-case for /proc/pid/clear_refs is 
in conjunction with /proc/pid/smaps for approximating memory footprint, 
we'll end up saving TLB flushes because the granularity with which that 
measurement is taken is usually very fine.

Acked-by: David Rientjes <rientjes@...gle.com>
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