linux-kernel - Re: copy_from_user / copy_to

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Message-Id: <B72905BB-6E8D-47FD-9A20-269B27136DB2@mac.com>
Date:	Mon, 16 Oct 2006 15:39:16 -0400
From:	Kyle Moffett <mrmacman_g4@....com>
To:	mfbaustx <mfbaustx@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: copy_from_user / copy_to_user with no swap space

On Oct 16, 2006, at 15:19:03, mfbaustx wrote:
> So you're absolutely obligated to DO the copy at the time the  
> kernel is executing on behalf of that process.  Once your process/ 
> thread is context swapped, you've lost the [correct] information on  
> the address mapping.

Yes, this is correct.

> So, IF you MUST copy_from/to_user when in the context of the  
> process, AND IF you have no virtual memory/swapping, THEN must it  
> not be true that you can ALWAYS dereferences your user space pointers?

I'm not sure I entirely understand what you're asking here; perhaps  
you could rephrase or explain what you're trying to do?  From what I  
can pick up from your description; you may be missing that program  
text pages and memory-mapped files may be "swapped-out" even  
*without* a swap device.  As an example, when I first start /bin/bash  
(ignoring readahead for the moment), very little of the binary and  
shared libraries are actually in memory (the rest is left on disk).   
When I use data or call a function that hasn't been loaded from disk  
yet, a major fault occurs, the kernel loads data from the bash  
executable file or a shared library, and then maps it into the  
process address space.

Cheers,
Kyle Moffett

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