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Date: Thu, 9 Sep 2010 09:48:13 +0200
From: Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH] x86: copy_oldmem_page using cached addressing
Message-ID: <20100909074813.GA8355@elte.hu>
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To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: kexec@lists.infradead.org, Cliff Wickman <cpw@sgi.com>, linux-kernel@vger.kernel.org


* Eric W. Biederman <ebiederm@xmission.com> wrote:

> Cliff Wickman <cpw@sgi.com> writes:
> 
> > From: Cliff Wickman <cpw@sgi.com>
> >
> > The copy of /proc/vmcore to a user buffer proceeds much faster
> > if the kernel addresses memory as cached.
> >
> > With this patch we have seen an increase in transfer rate from less than
> > 15MB/s to 80-460MB/s, depending on size of the transfer.  This makes
> > a big difference in time needed to save a system dump.
> >
> > (Does anyone know of a reason why copy_oldmem_page() would need
> >  to use uncached addresses?)
> >
> > Diffed against 2.6.36-rc3
> 
> I believe this code simply predates being able to specify the
> caching attributes at ioremap time.  Being cached in this case
> actually looks more correct, as that is the default for RAM
> and we are talking about RAM.
> 
> Ultimately either there is another cpu running wild with these pages
> mapped with some random set of attributes, and we cannot get the
> permissions right or we have captured all of the cpus and this is the
> only mapping so it doesn't matter.
> 
> Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

Applied, thanks guys. Note, i also added a -stable tag, as kdump is 
typically used with enterprise setups and we want this (low-risk, 
high-impact) improvement to be propagated far back the -stable series.

Thanks,

	Ingo

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