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Date: Mon, 29 Jan 2007 18:15:57 -0800
From: Andrew Morton <akpm@osdl.org>
To: "Martin J. Bligh" <mbligh@mbligh.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, Ingo Molnar <mingo@elte.hu>,
       David Chinner <dgc@sgi.com>
Subject: Re: [PATCH] mm: remove global locks from mm/highmem.c
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On Mon, 29 Jan 2007 17:49:14 -0800
"Martin J. Bligh" <mbligh@mbligh.org> wrote:

> Andrew Morton wrote:
> > On Mon, 29 Jan 2007 17:31:20 -0800
> > "Martin J. Bligh" <mbligh@mbligh.org> wrote:
> > 
> >> Peter Zijlstra wrote:
> >>> On Sun, 2007-01-28 at 14:29 -0800, Andrew Morton wrote:
> >>>
> >>>> As Christoph says, it's very much preferred that code be migrated over to
> >>>> kmap_atomic().  Partly because kmap() is deadlockable in situations where a
> >>>> large number of threads are trying to take two kmaps at the same time and
> >>>> we run out.  This happened in the past, but incidences have gone away,
> >>>> probably because of kmap->kmap_atomic conversions.
> >>>> From which callsite have you measured problems?
> >>> CONFIG_HIGHPTE code in -rt was horrid. I'll do some measurements on
> >>> mainline.
> >>>
> >> CONFIG_HIGHPTE is always horrid -we've known that for years.
> > 
> > We have?  What's wrong with it?  <looks around for bug reports>
> 
> http://www.ussg.iu.edu/hypermail/linux/kernel/0307.0/0463.html

2% overhead for a pte-intensive workload for unknown reasons four years
ago.  Sort of a mini-horrid, no?

We still don't know what is the source of kmap() activity which
necessitated this patch btw.  AFAIK the busiest source is ext2 directories,
but perhaps NFS under certain conditions?

<looks at xfs_iozero>

->prepare_write no longer requires that the caller kmap the page.
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