From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DEF37C48BDF for ; Tue, 22 Jun 2021 22:20:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C16A2601FE for ; Tue, 22 Jun 2021 22:20:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230107AbhFVWXG (ORCPT ); Tue, 22 Jun 2021 18:23:06 -0400 Received: from mail108.syd.optusnet.com.au ([211.29.132.59]:55046 "EHLO mail108.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229718AbhFVWXF (ORCPT ); Tue, 22 Jun 2021 18:23:05 -0400 Received: from dread.disaster.area (pa49-179-138-183.pa.nsw.optusnet.com.au [49.179.138.183]) by mail108.syd.optusnet.com.au (Postfix) with ESMTPS id 7E20B1B3100; Wed, 23 Jun 2021 08:20:45 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1lvolA-00FrD1-A6; Wed, 23 Jun 2021 08:20:44 +1000 Date: Wed, 23 Jun 2021 08:20:44 +1000 From: Dave Chinner To: David Laight Cc: 'David Howells' , Al Viro , "torvalds@linux-foundation.org" , Ted Ts'o , Dave Hansen , Andrew Morton , "willy@infradead.org" , "linux-mm@kvack.org" , "linux-ext4@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? Message-ID: <20210622222044.GI2419729@dread.disaster.area> References: <3221175.1624375240@warthog.procyon.org.uk> <3225322.1624379221@warthog.procyon.org.uk> <7a6d8c55749d46d09f6f6e27a99fde36@AcuMS.aculab.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7a6d8c55749d46d09f6f6e27a99fde36@AcuMS.aculab.com> X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=Tu+Yewfh c=1 sm=1 tr=0 a=MnllW2CieawZLw/OcHE/Ng==:117 a=MnllW2CieawZLw/OcHE/Ng==:17 a=v-Dl0aO_AE6Q_TjM:21 a=kj9zAlcOel0A:10 a=r6YtysWOX24A:10 a=drOt6m5kAAAA:8 a=7-415B0cAAAA:8 a=LpHeB8C7BEPH7Pa3heMA:9 a=CjuIK1q_8ugA:10 a=vIikcsq8ZuViU5wKlUpU:22 a=RMMjzBEyIzXRtoq5n5K6:22 a=biEYGPWJfzWAr4FL6Ov7:22 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue, Jun 22, 2021 at 09:55:09PM +0000, David Laight wrote: > From: David Howells > > Sent: 22 June 2021 17:27 > > > > Al Viro wrote: > > > > > On Tue, Jun 22, 2021 at 04:20:40PM +0100, David Howells wrote: > > > > > > > and wondering if the iov_iter_fault_in_readable() is actually effective. > > > > Yes, it can make sure that the page we're intending to modify is dragged > > > > into the pagecache and marked uptodate so that it can be read from, but is > > > > it possible for the page to then get reclaimed before we get to > > > > iov_iter_copy_from_user_atomic()? a_ops->write_begin() could potentially > > > > take a long time, say if it has to go and get a lock/lease from a server. > > > > > > Yes, it is. So what? We'll just retry. You *can't* take faults while > > > holding some pages locked; not without shitloads of deadlocks. > > > > In that case, can we amend the comment immediately above > > iov_iter_fault_in_readable()? > > > > /* > > * Bring in the user page that we will copy from _first_. > > * Otherwise there's a nasty deadlock on copying from the > > * same page as we're writing to, without it being marked > > * up-to-date. > > * > > * Not only is this an optimisation, but it is also required > > * to check that the address is actually valid, when atomic > > * usercopies are used, below. > > */ > > if (unlikely(iov_iter_fault_in_readable(i, bytes))) { > > > > The first part suggests this is for deadlock avoidance. If that's not true, > > then this should perhaps be changed. > > I'd say something like: > /* > * The actual copy_from_user() is done with a lock held > * so cannot fault in missing pages. > * So fault in the pages first. > * If they get paged out the inatomic usercopy will fail > * and the whole operation is retried. > * > * Hopefully there are enough memory pages available to > * stop this looping forever. > */ What about the other 4 or 5 copies of this loop in the kernel? This is a pattern, not a one off implementation. Comments describing how the pattern works belong in the API documentation, not on a single implemenation of the pattern... Cheers, Dave. -- Dave Chinner david@fromorbit.com