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Re: kernel BUG at fs/btrfs/volumes.c:2733

Ilya Dryomov wrote (ao):
> On Thu, Mar 29, 2012 at 12:52:35PM +0200, Sander wrote:
> > After a reboot, I tried again, with the same result:
> > 
> > [   81.048767] ------------[ cut here ]------------
> > [   81.053619] kernel BUG at fs/btrfs/volumes.c:2733!
> > [   81.053619] Internal error: Oops - BUG: 0 [#1] SMP
> > [   81.059295] Modules linked in:
> > [   81.059295] CPU: 1    Not tainted  (3.3.0 #8)
> > [   81.071411] PC is at btrfs_balance+0x312/0xb04
> > [   81.074890] LR is at btrfs_run_delayed_iputs+0x2d/0xac

> So you have balance item on disk, but the kernel doesn't seem to know
> about it in advance, which is odd and so when you try to run balance it
> panics on one of the safety checks.
> > The system is a pandaboard running a plain Linus kernel 3.3.0 with a
> > btrfs filesystem, over two Intel 320 600GB ssd's, connected via usb (on
> > an usb hub), on top of md_crypt. Mount options:
> > "subvol=rootvolume,space_cache,inode_cache,compress=lzo,ssd"
> > 
> > Before the balance, I deleted about 2500 snapshots and waited for the
> > btrfs kernel threads to calm down. Then I initiated a btrfs filesystem
> > scrub. Unfortunately during the scrub, the filesystem balance started.
> > Might be related.
> That's indeed pretty cool, I wonder how that could happen.

I create 5 snapshots of 5 different subvolumes every 5 minutes, and the
system is low on memory:

             total       used       free     shared    buffers     cached
Mem:           745        712         33          0          0        480
-/+ buffers/cache:        231        514
Swap:            0          0          0

There is ample space on the fileystem:
panda:~# df -h /
Filesystem                                              Size  Used Avail Use% Mounted on
/dev/mapper/ata-INTEL_SSDSA2CW600G3_CVPR112405AJ600FGN  1.1T   17G  1.1T   2% /

panda:~# btrfs filesystem df /
Data, RAID0: total=24.00GB, used=15.69GB
System, RAID1: total=64.00MB, used=12.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=23.00GB, used=231.26MB

Do you need more information?

> I'm definitely intrested in reproducing it. Could you please umount this
> filesystem, capture the output of 'btrfs-debug-tree -d <dev>' and post it
> somewhere ?

Will do. It is the / filesystem, so I'll need to reboot.

> After that mount it back and see if there is "btrfs: continuing
> balance" line in dmesg (and if btrfs-balance kthread shows up)?

There was none after the first reboot, but I'll pay extra attention to
that after the next reboot.

> If so, just let it run, it should finish the balance and remove
> on-disk item. (You can query the status of running balance with 'btrfs
> balance status <mnt>')

Do I need newer tools for that? This is Debian Sid (unstable):

Btrfs Btrfs v0.19

panda:~# btrfs filesystem balance status /
ERROR: 'filesystem balance' requires 1 arg(s)

panda:~# btrfs balance status /
ERROR: unknown command 'balance'

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