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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 CD639C2D0A2 for ; Fri, 16 Oct 2020 08:46:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8151D20848 for ; Fri, 16 Oct 2020 08:46:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404478AbgJPIqV (ORCPT ); Fri, 16 Oct 2020 04:46:21 -0400 Received: from mx2.suse.de ([195.135.220.15]:45314 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2394772AbgJPIqP (ORCPT ); Fri, 16 Oct 2020 04:46:15 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id D47D3ADE0; Fri, 16 Oct 2020 08:46:13 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 916201E133E; Fri, 16 Oct 2020 10:46:13 +0200 (CEST) Date: Fri, 16 Oct 2020 10:46:13 +0200 From: Jan Kara To: Dave Chinner Cc: Jan Kara , fdmanana@kernel.org, fstests@vger.kernel.org, linux-btrfs@vger.kernel.org, Filipe Manana Subject: Re: [PATCH] generic: test the correctness of several cases of RWF_NOWAIT writes Message-ID: <20201016084613.GJ7037@quack2.suse.cz> References: <20201015161355.GI7037@quack2.suse.cz> <20201016055757.GA7322@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201016055757.GA7322@dread.disaster.area> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Fri 16-10-20 16:57:57, Dave Chinner wrote: > On Thu, Oct 15, 2020 at 06:13:56PM +0200, Jan Kara wrote: > > On Thu 15-10-20 16:36:38, fdmanana@kernel.org wrote: > > > From: Filipe Manana > > > > > > Verify some attempts to write into a file using RWF_NOWAIT: > > > > > > 1) Writing into a fallocated extent that starts at eof should work; > > > > Why? We need to update i_size which requires transaction start and e.g. > > ext4 does not support that in non-blocking mode... > > Right, different filesystems behave differently given similar > pre-conditions. That's not a bug, that's exactly how RWF_NOWAIT is > expected to be implemented by each filesystem.... > > > > 2) Writing into a hole should fail; > > > 3) Writing into a range that is partially allocated should fail. > > Same for these - these are situations where a -specific filesystem > implementation- might block, not a situation where the RWF_NOWAIT > API specification says that IO submission "should fail" and hence > return EAGAIN. > > > > This is motivated by several bugs that btrfs and ext4 had and were fixed > > > by the following kernel commits: > > > > > > 4b1946284dd6 ("btrfs: fix failure of RWF_NOWAIT write into prealloc extent beyond eof") > > > 260a63395f90 ("btrfs: fix RWF_NOWAIT write not failling when we need to cow") > > > 0b3171b6d195 ("ext4: do not block RWF_NOWAIT dio write on unallocated space") > > > > > > At the moment, on a 5.9-rc6 kernel at least, ext4 is failing for case 1), > > > but when I found and fixed case 1) in btrfs, around kernel 5.7, it was not > > > failing on ext4, so some regression happened in the meanwhile. For xfs and > > > btrfs on a 5.9 kernel, all the three cases pass. > > Sure, until we propagate IOMAP_NOWAIT far enough into the allocation > code that allocation will either succeed without blocking or fail > without changing anything. At which point, the filesystem behaviour > is absolutely correct according to the RWF_NOWAIT specification, but > the test is most definitely wrong. > > IOWs, I think any test that says "RWF_NOWAIT IO in a situation> must do " is incorrect. RWF_NOWAIT simply > does not not define behaviour like this, and different filesystems > will do different things given the same file layouts... I agree with this. That being said it would be still worthwhile to have some tests verifying RWF_NOWAIT behavior is sane - that we don't block with RWF_NOWAIT (this is a requirement), and that what used to work with RWF_NOWAIT didn't unexpectedly regress (this is more a sanity check)... I'm not sure how to test that in an automated way through. Honza -- Jan Kara SUSE Labs, CR