From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([59.151.112.132]:57487 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1754259AbdD1Apk (ORCPT ); Thu, 27 Apr 2017 20:45:40 -0400 Subject: Re: File system corruption, btrfsck abort To: Christophe de Dinechin , References: From: Qu Wenruo Message-ID: <43eda9a8-e7be-4bc3-bd37-4df44f93f321@cn.fujitsu.com> Date: Fri, 28 Apr 2017 08:45:37 +0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8"; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: At 04/26/2017 01:50 AM, Christophe de Dinechin wrote: > Hi, > > > I”ve been trying to run btrfs as my primary work filesystem for about 3-4 months now on Fedora 25 systems. I ran a few times into filesystem corruptions. At least one I attributed to a damaged disk, but the last one is with a brand new 3T disk that reports no SMART errors. Worse yet, in at least three cases, the filesystem corruption caused btrfsck to crash. > > The last filesystem corruption is documented here: https://bugzilla.redhat.com/show_bug.cgi?id=1444821. The dmesg log is in there. According to the bugzilla, the btrfs-progs seems to be too old in btrfs standard. What about using the latest btrfs-progs v4.10.2? Furthermore for v4.10.2, btrfs check provides a new mode called lowmem. You could try "btrfs check --mode=lowmem" to see if such problem can be avoided. For the kernel bug, it seems to be related to wrongly inserted delayed ref, but I can totally be wrong. Thanks, Qu > > The btrfsck crash is here: https://bugzilla.redhat.com/show_bug.cgi?id=1435567. I have two crash modes: either an abort or a SIGSEGV. I checked that both still happens on master as of today. > > The cause of the abort is that we call set_extent_dirty from check_extent_refs with rec->max_size == 0. I’ve instrumented to try to see where we set this to 0 (see https://github.com/c3d/btrfs-progs/tree/rhbz1435567), and indeed, we do sometimes see max_size set to 0 in a few locations. My instrumentation shows this: > > 78655 [1.792241:0x451fe0] MAX_SIZE_ZERO: Add extent rec 0x139eb80 max_size 16384 tmpl 0x7fffffffd120 > 78657 [1.792242:0x451cb8] MAX_SIZE_ZERO: Set max size 0 for rec 0x139ec50 from tmpl 0x7fffffffcf80 > 78660 [1.792244:0x451fe0] MAX_SIZE_ZERO: Add extent rec 0x139ed50 max_size 16384 tmpl 0x7fffffffd120 > > I don’t really know what to make of it. > > The cause of the SIGSEGV is that we try to free a list entry that has its next set to NULL. > > #0 list_del (entry=0x555555db0420) at /usr/src/debug/btrfs-progs-v4.10.1/kernel-lib/list.h:125 > #1 free_all_extent_backrefs (rec=0x555555db0350) at cmds-check.c:5386 > #2 maybe_free_extent_rec (extent_cache=0x7fffffffd990, rec=0x555555db0350) at cmds-check.c:5417 > #3 0x00005555555b308f in check_block (flags=, buf=0x55557b87cdf0, extent_cache=0x7fffffffd990, root=0x55555587d570) at cmds-check.c:5851 > #4 run_next_block (root=root@entry=0x55555587d570, bits=bits@entry=0x5555558841 > > I don’t know if the two problems are related, but they seem to be pretty consistent on this specific disk, so I think that we have a good opportunity to improve btrfsck to make it more robust to this specific form of corruption. But I don’t want to hapazardly modify a code I don’t really understand. So if anybody could make a suggestion on what the right strategy should be when we have max_size == 0, or how to avoid it in the first place. > > I don’t know if this is relevant at all, but all the machines that failed that way were used to run VMs with KVM/QEMU. DIsk activity tends to be somewhat intense on occasions, since the VMs running there are part of a personal Jenkins ring that automatically builds various projects. Nominally, there are between three and five guests running (Windows XP, WIndows 10, macOS, Fedora25, Ubuntu 16.04). > > > Thanks > Christophe de Dinechin > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > >