linux-kernel - Re: how to tell if arbitrary kernel memory address is backed by physical memory?

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Message-ID: <49E8AB11.4000708@nortel.com>
Date:	Fri, 17 Apr 2009 10:15:13 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: how to tell if arbitrary kernel memory address is backed by physical
 memory?

Christoph Lameter wrote:
> On Thu, 16 Apr 2009, Chris Friesen wrote:

>> Is there a portable way to tell whether a particular virtual address in the
>> lowmem address range is backed by physical memory and is readable?
>>
>> For background...we have some guys working on a software memory scrubber for
>> an embedded board.  The memory controller supports ECC but doesn't support
>> scrubbing  in hardware.  What we want to do is walk all of lowmem, reading in
>> memory.  If a fault is encountered, it will be handled by other code.
> 
> Virtual address in the lowmem address range? lowmem address ranges exist
> for physical addresses.
> 
> If you walk lowmem (physical) then you will never see a missing page.

We have a mips board that appears to have holes in the lowmem mappings 
such that blindly walking all of it causes problems.  I assume the 
allocator knows about these holes and simply doesn't assign memory at 
those addresses.

We may have found a solution though...it looks like virt_addr_valid() 
returns false for the problematic addresses.  Would it be reasonable to 
call this once for each page before trying to access it?

Chris

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