[PATCH] x86: copy_oldmem_page using cached addressing

Ingo Molnar mingo at elte.hu
Thu Sep 9 03:48:13 EDT 2010

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

> Cliff Wickman <cpw at sgi.com> writes:
> > From: Cliff Wickman <cpw at 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 at 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.



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