From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail02.iobjects.de ([188.40.134.68]:59738 "EHLO mail02.iobjects.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933480AbcKJPLi (ORCPT ); Thu, 10 Nov 2016 10:11:38 -0500 Subject: Re: [PATCH] Btrfs: deal with existing encompassing extent map in btrfs_get_extent() To: Omar Sandoval , linux-btrfs@vger.kernel.org References: <262a1e171d091626edbd23c637cb138ba9d84ed8.1478733376.git.osandov@fb.com> Cc: kernel-team@fb.com From: =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= Message-ID: <58248E27.7080601@applied-asynchrony.com> Date: Thu, 10 Nov 2016 16:11:35 +0100 MIME-Version: 1.0 In-Reply-To: <262a1e171d091626edbd23c637cb138ba9d84ed8.1478733376.git.osandov@fb.com> Content-Type: text/plain; charset=utf-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 11/10/16 00:26, Omar Sandoval wrote: > From: Omar Sandoval > > My QEMU VM was seeing inexplicable I/O errors that I tracked down to > errors coming from the qcow2 virtual drive in the host system. The qcow2 > file is a nocow file on my Btrfs drive, which QEMU opens with O_DIRECT. > Every once in awhile, pread() or pwrite() would return EEXIST, which > makes no sense. This turned out to be a bug in btrfs_get_extent(). > > Commit 8dff9c853410 ("Btrfs: deal with duplciates during extent_map > insertion in btrfs_get_extent") fixed a case in btrfs_get_extent() where > two threads race on adding the same extent map to an inode's extent map > tree. However, if the added em is merged with an adjacent em in the > extent tree, then we'll end up with an existing extent that is not > identical to but instead encompasses the extent we tried to add. When we > call merge_extent_mapping() to find the nonoverlapping part of the new > em, the arithmetic overflows because there is no such thing. We then end > up trying to add a bogus em to the em_tree, which results in a EEXIST > that can bubble all the way up to userspace. > > Fix it by extending the identical extent map special case. > > Signed-off-by: Omar Sandoval > --- > Applies to 4.9-rc4. > > Here [1] is a reproducer for this bug that doesn't involve firing up a > QEMU VM. Also, a big shoutout to BCC [2] and BPF for making it possible > to debug this on my laptop without compiling a custom kernel and > rebooting just to add printks [3]. > > 1: https://gist.github.com/osandov/d08aabe5d4dec15517e9fde17012fd3b I can't really make this reproducer fail. It builds and runs fine, but just exits with no messages (other than the one about drop_caches in dmesg). It creates the 1MB file and always returns 0. Ideas? -h