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Message-ID: <20131.1303319102@jupiter.eclipse.co.uk>
To: "Rik van Riel" <riel@redhat.com>, <rwhitton@iee.org>
Reply-To: rwhitton@iee.org
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Date: Wed, 20 Apr 2011 18:05:02 +0100
Subject: Re: Background memory scrubbing
From: Robert Whitton <rwhitton@iee.org>
Cc: <linux-kernel@vger.kernel.org>
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On Wed 20/04/11  6:45 PM , Rik van Riel <riel@redhat.com> wrote:

> On 04/20/2011 03:58 AM, Robert Whitton wrote:
> 
> > for each PFN from 256 to the highest valid PFN
> > {
> > if (pfn_valid(PFN))
> > {
> > page = pfn_to_page(PFN)
> > va = kmap(page)
> > atomic_scrub(va, PAGE_SIZE)
> > kunmap(page)
> > }
> >
> > sleep(for_a_while)
> > }
> 
> What exactly does atomic_scrub do?

atomic_scrub is part of the edac subsystem see arch/x86/include/asm/edac.h. It simply does a locked add of zero to each DWORD in the specified range.

(a shame that for 64 bit platforms it doesn't use QWORDS but that's just an optimisation)

> 
> > This code works absolutely fine up to a short distance beyond the 16MB
> boundary (specifically it seems to always fail on my hardware at PFN
> 4105). At this point despite the fact that kmap returns a valid virtual
> address (and it is the virtual address that I expect - 0xffff880001009000)
> I get the kernel oops - "unable to handle kernel paging request".
> 
> Looks like you might be making some of the kernel code that
> is running at that moment unreachable, leading to a kernel
> page fault.
> 
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