From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752931AbZAKJz6 (ORCPT ); Sun, 11 Jan 2009 04:55:58 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751320AbZAKJzu (ORCPT ); Sun, 11 Jan 2009 04:55:50 -0500 Received: from wilson.telenet-ops.be ([195.130.132.42]:58735 "EHLO wilson.telenet-ops.be" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751200AbZAKJzs (ORCPT ); Sun, 11 Jan 2009 04:55:48 -0500 Date: Sun, 11 Jan 2009 10:55:45 +0100 (CET) From: Geert Uytterhoeven To: Ingo Molnar cc: Peter Zijlstra , Brad Parker , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: copy_{to,from}_user In-Reply-To: <20090111020923.GC29153@elte.hu> Message-ID: References: <4056.1231523557@mini> <1231538061.29452.8.camel@twins> <20090111020923.GC29153@elte.hu> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 11 Jan 2009, Ingo Molnar wrote: > * Peter Zijlstra wrote: > > 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. > > you sure must have meant 'write 7 words' or something like that? I understood it as: the code read 8 words, and wrote 8 words. But only n < 8 words could be written, while the code still returned 8. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds