From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758030AbZAIVy0 (ORCPT ); Fri, 9 Jan 2009 16:54:26 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751908AbZAIVyS (ORCPT ); Fri, 9 Jan 2009 16:54:18 -0500 Received: from bombadil.infradead.org ([18.85.46.34]:42172 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751659AbZAIVyR (ORCPT ); Fri, 9 Jan 2009 16:54:17 -0500 Subject: Re: copy_{to,from}_user From: Peter Zijlstra To: Brad Parker Cc: linux-kernel@vger.kernel.org, Linus Torvalds , Ingo Molnar In-Reply-To: <4056.1231523557@mini> References: <4056.1231523557@mini> Content-Type: text/plain Content-Transfer-Encoding: 7bit Date: Fri, 09 Jan 2009 22:54:21 +0100 Message-Id: <1231538061.29452.8.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.24.2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.