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=-5.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 083C4C2D0B1 for ; Tue, 4 Feb 2020 13:58:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DD83D2082E for ; Tue, 4 Feb 2020 13:58:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727260AbgBDN6y (ORCPT ); Tue, 4 Feb 2020 08:58:54 -0500 Received: from mail.itouring.de ([188.40.134.68]:53586 "EHLO mail.itouring.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727207AbgBDN6y (ORCPT ); Tue, 4 Feb 2020 08:58:54 -0500 Received: from tux.applied-asynchrony.com (p5B07E981.dip0.t-ipconnect.de [91.7.233.129]) by mail.itouring.de (Postfix) with ESMTPSA id 1376D4160454; Tue, 4 Feb 2020 14:58:53 +0100 (CET) Received: from [192.168.100.223] (ragnarok.applied-asynchrony.com [192.168.100.223]) by tux.applied-asynchrony.com (Postfix) with ESMTP id BCA65F01602; Tue, 4 Feb 2020 14:58:52 +0100 (CET) Subject: Re: Kernels 4.15..5.5: "WARNING: CPU: 2 PID: 4150 at fs/fs-writeback.c:2363 __writeback_inodes_sb_nr+0xa9/0xc0" To: Zygo Blaxell , linux-btrfs@vger.kernel.org References: <20190322041731.GF16651@hungrycats.org> <20200204050456.GB13306@hungrycats.org> From: =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= Organization: Applied Asynchrony, Inc. Message-ID: <65514978-506f-83fa-2c95-ee9ce3cbf5b4@applied-asynchrony.com> Date: Tue, 4 Feb 2020 14:58:52 +0100 MIME-Version: 1.0 In-Reply-To: <20200204050456.GB13306@hungrycats.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 2/4/20 6:04 AM, Zygo Blaxell wrote: > On Fri, Mar 22, 2019 at 12:17:32AM -0400, Zygo Blaxell wrote: >> When filesystems are mounted flushoncommit, I get this warning roughly >> every 30 seconds: >> >> [ 4575.142805] WARNING: CPU: 3 PID: 4150 at fs/fs-writeback.c:2363 __writeback_inodes_sb_nr+0xa9/0xc0 >> [ 4575.145567] Modules linked in: crct10dif_pclmul crc32_pclmul dm_cache_smq crc32c_intel dm_cache snd_pcm ghash_clmulni_intel aesni_intel sr_mod dm_persistent_data ppdev joydev dm_bio_prison aes_x86_64 crypto_simd snd_timer dm_bufio cryptd cdrom snd glue_helper dm_mod parport_pc soundcore sg floppy parport pcspkr psmouse bochs_drm rtc_cmos ide_pci_generic piix input_leds i2c_piix4 ide_core serio_raw evbug qemu_fw_cfg evdev ip_tables x_tables ipv6 crc_ccitt autofs4 >> [ 4575.160021] CPU: 3 PID: 4150 Comm: btrfs-transacti Tainted: G W 5.0.3-zb64+ #1 >> [ 4575.162484] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014 >> [ 4575.164505] RIP: 0010:__writeback_inodes_sb_nr+0xa9/0xc0 >> [ 4575.165809] Code: 0f b6 d2 e8 b9 f8 ff ff 48 89 ee 48 89 df e8 0e f8 ff ff 48 8b 44 24 48 65 48 33 04 25 28 00 00 00 75 0b 48 83 c4 50 5b 5d c3 <0f> 0b eb cb e8 4e e9 d6 ff 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 >> [ 4575.171927] RSP: 0018:ffffa9cac0eabde8 EFLAGS: 00010246 >> [ 4575.173045] RAX: 0000000000000000 RBX: ffff9353e23af000 RCX: 0000000000000000 >> [ 4575.175639] RDX: 0000000000000002 RSI: 0000000000030c67 RDI: ffffa9cac0eabe30 >> [ 4575.177619] RBP: ffffa9cac0eabdec R08: ffffa9cac0eabdf0 R09: ffff9353f12da000 >> [ 4575.179736] R10: 0000000000000000 R11: 0000000000000001 R12: ffff9353e1980000 >> [ 4575.181661] R13: ffff9353e1981430 R14: ffff9353f27e4260 R15: ffff9353e1981518 >> [ 4575.183871] FS: 0000000000000000(0000) GS:ffff9353f6800000(0000) knlGS:0000000000000000 >> [ 4575.185940] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> [ 4575.188072] CR2: 00007fb81841fa20 CR3: 00000002218c0006 CR4: 00000000001606e0 >> [ 4575.190094] Call Trace: >> [ 4575.190828] btrfs_commit_transaction+0x7a6/0x9e0 >> [ 4575.192115] ? start_transaction+0x91/0x4d0 >> [ 4575.193197] transaction_kthread+0x146/0x180 >> [ 4575.194415] kthread+0x106/0x140 >> [ 4575.195403] ? btrfs_cleanup_transaction+0x620/0x620 >> [ 4575.196903] ? kthread_park+0x90/0x90 >> [ 4575.198412] ret_from_fork+0x3a/0x50 >> [ 4575.199374] irq event stamp: 54922780 >> [ 4575.200218] hardirqs last enabled at (54922779): [] _raw_spin_unlock_irqrestore+0x32/0x60 >> [ 4575.202753] hardirqs last disabled at (54922780): [] trace_hardirqs_off_thunk+0x1a/0x1c >> [ 4575.205921] softirqs last enabled at (54922378): [] __do_softirq+0x3a4/0x45f >> [ 4575.208350] softirqs last disabled at (54922361): [] irq_exit+0xe4/0xf0 >> [ 4575.210616] ---[ end trace 5309dcf3a1920eca ]--- >> >> For my own kernel builds I just comment out the line in fs-writeback.c, >> but that's not a real solution. > > This still happens in 5.5.0. No changes in behavior or workaround, no > apparent harmful effect, almost 2 years running in stress-testing and > production. > > I, for one, am glad we fixed all those other bugs before doing anything > about this one. It is utterly harmless. This triggered my archeology itch. I had to go deeper. The warning goes all the way back to 2010 (kernel 2.6.x) when everything happened at FusionIO. Commit [1] introduced it as preparation for [2]. The only caller of writeback_inodes_sb_nr is btrfs_writeback_inodes_sb_nr in (today's) space-info.c, where the mutex trylock was introduced in [3], apparently to work around a VFS function that didn't do it for btrfs at the time. Flushoncommit was added by Sage Weil for Ceph's btrfs backend in [4], even before the WARN_ON, in 2009. We know how that story ended. Why has nobody except you noticed this? Probably because the number of people actually using it or reporting bugs is.. very small. ¯\_(ツ)_/¯ Unfortunately I'm still none the wiser why btrfs feels it's necessary to "open-code"/circumvent the rwsem check. Maybe this gives you a clue. cheers, Holger [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/fs-writeback.c?id=cf37e972478ec58a8a54a6b4f951815f0ae28f78 [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/fs-writeback.c?id=d19de7edf59cdd586777b009e0e8fbe5412dd35f [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/btrfs/extent-tree.c?id=925a6efb8ff0c2bdbec107ed9890e62650c83306 [4] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=dccae99995089641fbac452ebc7f0cab18751ddb