linux-ext4 - Re: [4.7-rc6 ext3 corruption] ext4_mb_generate_buddy:758: group 27, block bitmap and bg descriptor inconsistent:

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Message-ID: <20160727154843.GB1659@quack2.suse.cz>
Date:	Wed, 27 Jul 2016 17:48:43 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [4.7-rc6 ext3 corruption] ext4_mb_generate_buddy:758: group 27,
 block bitmap and bg descriptor inconsistent:

Hi!

On Tue 12-07-16 15:41:37, Dave Chinner wrote:
> Just rebooted a 4.7-rc6 test VM, and the root filesystem had the
> journal abort a couple of seconds after mount while the system was
> still booting:
> 
> [    3.043543] EXT4-fs (sda1): mounting ext3 file system using the ext4 subsystem
> [    3.045027] EXT4-fs (sda1): INFO: recovery required on readonly filesystem
> [    3.046008] EXT4-fs (sda1): write access will be enabled during recovery
> [    3.120052] EXT4-fs (sda1): recovery complete
> [    3.121746] EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: (null)
> [    3.122778] VFS: Mounted root (ext3 filesystem) readonly on device 8:1.
> .....
> [    5.263329] EXT4-fs error (device sda1): ext4_mb_generate_buddy:758: group 27, block bitmap and bg descriptor inconsistent: 4197 vs 4196 free clusters
> [    5.266343] Aborting journal on device sda1-8.
> [    5.267939] EXT4-fs (sda1): Remounting filesystem read-only
> [    5.269129] EXT4-fs error (device sda1) in ext4_free_blocks:4904: Journal has aborted
> [    5.271431] EXT4-fs error (device sda1) in ext4_do_update_inode:4891: Journal has aborted
> [    5.273720] EXT4-fs error (device sda1) in ext4_truncate:4150: IO failure
> [    5.275917] EXT4-fs error (device sda1) in ext4_orphan_del:2923: Journal has aborted
> [    5.278325] EXT4-fs error (device sda1) in ext4_do_update_inode:4891: Journal has aborted
> 
> The root filesystem checked clean three reboots before this
> occurred. e2fsck output on ro mounted fs:
> 
> # e2fsck /dev/sda1
> e2fsck 1.43-WIP (18-May-2015)
> /dev/sda1: recovering journal
> Superblock last mount time is in the future.
>         (by less than a day, probably due to the hardware clock being incorrectly set)
> /dev/sda1 contains a file system with errors, check forced.
> Pass 1: Checking inodes, blocks, and sizes
> Pass 2: Checking directory structure
> Pass 3: Checking directory connectivity
> Pass 4: Checking reference counts
> Pass 5: Checking group summary information
> Free blocks count wrong (542319, counted=546517).
> Fix<y>? yes
> Inode bitmap differences:  -219131
> Fix<y>? yes
> Free inodes count wrong for group #27 (6720, counted=6721).
> Fix<y>? yes
> Directories count wrong for group #27 (9, counted=8).
> Fix<y>? yes
> Free inodes count wrong (341015, counted=341018).
> Fix<y>? yes
> 
> /dev/sda1: ***** FILE SYSTEM WAS MODIFIED *****
> /dev/sda1: ***** REBOOT LINUX *****
> /dev/sda1: 283606/624624 files (3.1% non-contiguous), 1949574/2496091 blocks
> #

Hum, interesting. So 'Free blocks count wrong' and 'Free inodes count
wrong' messages are harmless - those entries and updated only
opportunistically and on mount and generally do not have to match on live
filesystem. The other three errors regarding inode and directory count are
a fallout from aborted inode deletion. Most importantly there is *no
problem* whatsoever with block bitmaps. So it was either some memory glitch
(bitflip in the counter or the bitmap) or there is some race and bb_free
can get out of sync with the bitmap and I don't see how that could happen
especially so early after mount... Strange.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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