linux-kernel - Re: copy_{to,from}

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Message-Id: <1231538061.29452.8.camel@twins>
Date:	Fri, 09 Jan 2009 22:54:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Brad Parker <brad@...ltoe.com>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: copy_{to,from}_user

On Fri, 2009-01-09 at 12:52 -0500, Brad Parker wrote:
> I have a question about copy_{to,from}_user.  
> 
> Most implementations I've seen do in-order copies and notice when an
> exception occurs and report back the progress.  This is straight
> forward.
> 
> (but to be honest, I have suspicions about how just how accurate those
> reports are i.e. +/- 1-3 bytes on some architectures)
> 
> On some cpu's it is advantageous to do an out-of-order copy to take
> advantage of various cache fill mechanisms.
> 
> The problem is that the out-of-order copy makes it impossible to know
> where the exception occurred (in terms of progress).
> 
> Would it be permissible to have a version of copy_{to,from}_user which
> does an out-of-order copy and when an exception occurs, restarts the
> copy from the beginning using a simple in-order copy, to make it
> possible to identify where the exception occurs?
> 
> The idea is that exceptions are rare and so the performance hit of doing
> the "recopy" would be minimal and would provide the required accuracy.

x86_64 already does some unrolling and is inaccurate as to where exactly
it happens. The only thing that is very important is that you _never_
say you copied more than you actually did.

That was the source of a data corruption bug a while ago, the code did
something like sequences: read 8 words, write 8 words. And reported the
number of bytes read, instead of bytes written, which is an
over-estimation.
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