From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cn.fujitsu.com ([59.151.112.132]:12076 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-FAIL) by vger.kernel.org with ESMTP id S1757437AbbIVBhI convert rfc822-to-8bit (ORCPT ); Mon, 21 Sep 2015 21:37:08 -0400 Subject: Re: kernel BUG at linux-4.2.0/fs/btrfs/extent-tree.c:1833 on rebalance To: =?UTF-8?Q?St=c3=a9phane_Lesimple?= References: <9c864637fe7676a8b7badc5ddd7a4e0c@all.all> <55F9486F.4040302@googlemail.com> <0973de930ee87e102c533c719807b748@all.all> <55FA2D9A.1060405@cn.fujitsu.com> <55FA60C5.5090002@cn.fujitsu.com> <7a6f2d794fb6cbf7d598b92e3470201c@all.all> <55FA759E.6030707@cn.fujitsu.com> <3386a8bfa1a5796460306a53a668e47e@all.all> <55FA98D8.5010301@gmx.com> <53a5553a9c5301789e246144bb264e43@all.all> <55FB61E9.4000300@cn.fujitsu.com> <2ce9b35f73732b145e0f80b18f230a52@all.all> <762ec73d5389b5057be4d3c17f74e1f9@all.all> <55FE0A50.9060607@gmx.com> <3ba27cf5afd82cf4e3bde718386b7cc3@all.all> <55FE8FB6.4070509@gmx.com> <72b4368e7180a4d703ef3ea1112d7358@all.all> <4749d42363070fcd228af172781750df@all.all> CC: Qu Wenruo , From: Qu Wenruo Message-ID: <5600B0BF.604@cn.fujitsu.com> Date: Tue, 22 Sep 2015 09:37:03 +0800 MIME-Version: 1.0 In-Reply-To: <4749d42363070fcd228af172781750df@all.all> Content-Type: text/plain; charset="utf-8"; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: Stéphane Lesimple wrote on 2015/09/22 03:30 +0200: > Le 2015-09-20 13:14, Stéphane Lesimple a écrit : >> Le 2015-09-20 12:51, Qu Wenruo a écrit : >>>>> Would you please use gdb to show the codes of >>>>> "btrfs_qgroup_rescan_worker+0x388" ? >>>>> (Need kernel debuginfo) >>>>> >>>>> My guess is the following line:(pretty sure, but not 100% sure) >>>>> ------ >>>>> /* >>>>> * only update status, since the previous part has alreay >>>>> updated the >>>>> * qgroup info. >>>>> */ >>>>> trans = btrfs_start_transaction(fs_info->quota_root, 1); <<<<< >>>>> if (IS_ERR(trans)) { >>>>> err = PTR_ERR(trans); >>>>> btrfs_err(fs_info, >>>>> "fail to start transaction for status >>>>> update: %d\n", >>>>> err); >>>>> goto done; >>>>> } >>>>> ------ >>>> >>>> The kernel and modules were already compiled with debuginfo. >>>> However for some reason, I couldn't get gdb disassembly of /proc/kcore >>>> properly >>>> aligned with the source I compiled: the asm code doesn't match the C >>>> code shown >>>> by gdb. In any case, watching the source of this function, this is the >>>> only place >>>> btrfs_start_transaction is called, so we can be 100% sure it's where >>>> the >>>> crash >>>> happens indeed. >>> >>> Yep, that's the only caller. >>> >>> Here is some useful small hint to locate the code, if you are >>> interestied in kernel development. >>> >>> # Not sure about whether ubuntu gzipped modules, at least Arch does >>> # compress it >>> $ cp /kernel/fs/btrfs/btrfs.ko.gz /tmp/ >>> $ gunzip /tmp/btrfs.ko.gz >>> $ gdb /tmp/btrfs.ko >>> # Make sure gdb read all the needed debuginfo >>> $ gdb list *(btrfs_qgroup_rescan_worker+0x388) >>> >>> And gdb will find the code position for you. >>> Quite easy one, only backtrace info is needed. >> >> Ah, thanks for the tips, I was loading whole vmlinux and using >> /proc/kcore >> as the core info, then adding the module with "add-symbol-file". But as >> we're just looking for the code and not the variables, it was indeed >> completely overkill. >> >> (gdb) list *(btrfs_qgroup_rescan_worker+0x388) >> 0x98068 is in btrfs_qgroup_rescan_worker (fs/btrfs/qgroup.c:2328). >> 2323 >> 2324 /* >> 2325 * only update status, since the previous part has >> alreay updated the >> 2326 * qgroup info. >> 2327 */ >> 2328 trans = btrfs_start_transaction(fs_info->quota_root, 1); >> 2329 if (IS_ERR(trans)) { >> 2330 err = PTR_ERR(trans); >> 2331 btrfs_err(fs_info, >> 2332 "fail to start transaction for >> status update: %d\n", >> >> So this just confirms what we were already 99% sure of. >> >>> Another hint is about how to collect the kernel crash info. >>> Your netconsole setup would be definitely one good practice. >>> >>> Another one I use to collect crash info is kdump. >>> Ubuntu should have a good wiki on it. >> >> I've already come across kdump a few times, but never really look into >> it. >> To debug the other complicated extend backref bug, it could be of some >> use. >> >>>>>>> So, as a quick summary of this big thread, it seems I've been >>>>>>> hitting >>>>>>> 3 bugs, all reproductible : >>>>>>> - kernel BUG on balance (this original thread) >>>>> >>>>> For this, I can't provide much help, as extent backref bug is quite >>>>> hard to debug, unless a developer is interested in it and find a >>>>> stable way to reproduce it. >>>> >>>> Yes, unfortunately as it looks so much like a race condition, I know >>>> I can >>>> reproduce it with my worflow, but it can take between 1 minute and 12 >>>> hours, >>>> so I wouldn't call it a "stable way" to reproduce it unfortunately :( >>>> >>>> Still if any dev is interested in it, I can reproduce it, with a >>>> patched >>>> kernel if needed. >>> >>> Maybe you are already doing it, you can only compile the btrfs >>> modules, which will be far more faster than compile the whole kernel, >>> if and only if the compiled module can be loaded. >> >> Yes, I've compiled this 4.3.0-rc1 in a completely modular form, so >> I'll try to >> load the modified module and see if the running kernel accepts it. I >> have to rmmod >> the loaded module first, hence umounting any btrfs fs before that. >> Should be able >> to do it in a couple hours. >> >> I'll delete again all my snapshots and run my script. Should be easy >> to trigger >> the (hopefully worked-around) bug again. > > Well, I didn't trigger this exact bug, but another one, not less severe > though, as it also crashed the system: > > [92098.841309] general protection fault: 0000 [#1] SMP > [92098.841338] Modules linked in: ... > [92098.841814] CPU: 1 PID: 24655 Comm: kworker/u4:12 Not tainted > 4.3.0-rc1 #1 > [92098.841834] Hardware name: ASUS All Series/H87I-PLUS, BIOS 1005 > 01/06/2014 > [92098.841868] Workqueue: btrfs-qgroup-rescan btrfs_qgroup_rescan_helper > [btrfs] > [92098.841889] task: ffff8800b6cc4100 ti: ffff8800a3dc8000 task.ti: > ffff8800a3dc8000 > [92098.841910] RIP: 0010:[] [] > memcpy_erms+0x6/0x10 > [92098.841935] RSP: 0018:ffff8800a3dcbcc8 EFLAGS: 00010207 > [92098.841950] RAX: ffff8800a3dcbd67 RBX: 0000000000000009 RCX: > 0000000000000009 > [92098.841970] RDX: 0000000000000009 RSI: 0005080000000000 RDI: > ffff8800a3dcbd67 > [92098.841989] RBP: ffff8800a3dcbd00 R08: 0000000000019c60 R09: > ffff88011fb19c60 > [92098.842009] R10: ffffea0003006480 R11: 0000000001000000 R12: > ffff8800b76c32c0 > [92098.842028] R13: 0000160000000000 R14: ffff8800a3dcbd70 R15: > 0000000000000009 > [92098.842048] FS: 0000000000000000(0000) GS:ffff88011fb00000(0000) > knlGS:0000000000000000 > [92098.842070] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [92098.842086] CR2: 00007fe1f2bd8000 CR3: 0000000001c10000 CR4: > 00000000000406e0 > [92098.842105] Stack: > [92098.842111] ffffffffc035a5d8 ffffffffc0396d00 000000000000028b > 0000000000000000 > [92098.842212] 0000cc6c00000000 ffff8800b76c3200 0000160000000000 > ffff8800a3dcbdc0 > [92098.842237] ffffffffc039af3d ffff8800c7196dc8 ffff8800c7196e08 > ffff8800c7196da0 > [92098.842261] Call Trace: > [92098.842277] [] ? read_extent_buffer+0xb8/0x110 > [btrfs] > [92098.842304] [] ? btrfs_find_all_roots+0x60/0x70 > [btrfs] > [92098.842329] [] > btrfs_qgroup_rescan_worker+0x28d/0x5a0 [btrfs] Would you please show the code of it? This one seems to be another stupid bug I made when rewriting the framework. Maybe I forgot to reinit some variants or I'm screwing memory... Thanks, Qu > [92098.842351] [] ? > ttwu_do_activate.constprop.90+0x5d/0x70 > [92098.842377] [] normal_work_helper+0xc0/0x270 [btrfs] > [92098.842401] [] > btrfs_qgroup_rescan_helper+0x12/0x20 [btrfs] > [92098.842421] [] process_one_work+0x14e/0x3d0 > [92098.842438] [] worker_thread+0x11a/0x470 > [92098.842454] [] ? rescuer_thread+0x310/0x310 > [92098.842471] [] kthread+0xc9/0xe0 > [92098.842485] [] ? kthread_park+0x60/0x60 > [92098.842502] [] ret_from_fork+0x3f/0x70 > [92098.842517] [] ? kthread_park+0x60/0x60 > [92098.842532] Code: ff eb eb 90 90 eb 1e 0f 1f 00 48 89 f8 48 89 d1 48 > c1 e9 03 83 e2 07 f3 48 a5 89 d1 f3 a4 c3 66 0f 1f 44 00 00 48 89 f8 48 > 89 d1 a4 c3 0f 1f 80 00 00 00 00 48 89 f8 48 83 fa 20 72 7e 40 38 > [92098.842658] RIP [] memcpy_erms+0x6/0x10 > [92098.842675] RSP > [92098.849594] ---[ end trace 9d5fb7931a3ec713 ]--- > > I would definitely say that rescans should be avoided on current kernels > as the possibility that it'll bring the system down shouldn't be ignored. > It confirms that this code really needs a rewrite ! > > Regards, >