Message-ID: <4DAF1DA7.9040705@redhat.com>
Date: Wed, 20 Apr 2011 13:53:43 -0400
From: Rik van Riel <riel@redhat.com>
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To: rwhitton@iee.org
CC: linux-kernel@vger.kernel.org
Subject: Re: Background memory scrubbing
References: <20131.1303319102@jupiter.eclipse.co.uk>
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On 04/20/2011 01:05 PM, Robert Whitton wrote:
> 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.

I can think of only a few ways in which that could cause a
kernel page fault.

One of the more obvious causes would be running into an
area of kernel memory that is mapped read-only.  Writing
to a page that is mapped read-only would cause a page
fault :)

Walking the page tables to check whether my guess is correct
should be possible in the current context.  Look at current->mm->pgd for 
the page directory and start walking from there.

Incidentally, the kernel mappings should be the same for any
process, so the above should hold true from any context.
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