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Date: Tue, 30 Jan 2007 17:11:32 -0800
From: Andrew Morton <akpm@osdl.org>
To: David Chinner <dgc@sgi.com>
Cc: "Martin J. Bligh" <mbligh@mbligh.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH] mm: remove global locks from mm/highmem.c
Message-Id: <20070130171132.7be3b054.akpm@osdl.org>
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On Wed, 31 Jan 2007 11:44:36 +1100
David Chinner <dgc@sgi.com> wrote:

> On Mon, Jan 29, 2007 at 06:15:57PM -0800, Andrew Morton wrote:
> > 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.
> 
> Agreed, but don't we (xfs_iozero) have to map it first to zero it?
> 
> I think what you are saying here, Andrew, is that we can
> do something like:
> 
> 	page = grab_cache_page
> 	->prepare_write(page)
> 	kaddr = kmap_atomic(page, KM_USER0)
> 	memset(kaddr+offset, 0, bytes)
> 	flush_dcache_page(page)
> 	kunmap_atomic(kaddr, KM_USER0)
> 	->commit_write(page)
> 
> to avoid using kmap() altogether?
> 

Yup.  Even better, use clear_highpage().
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