Re: kernel BUG at linux-4.2.0/fs/btrfs/extent-tree.c:1833 on rebalance

["Stéphane Lesimple" <stephane_btrfs@lesimple.fr>]

Le 2015-09-17 08:29, Stéphane Lesimple a écrit :
> Le 2015-09-16 15:04, Stéphane Lesimple a écrit :
>> I also disabled quota because it has almost for sure nothing
>> to do with the bug
> 
> As it turns out, it seems that this assertion was completely wrong.
> 
> I've got balance running for more than 16 hours now, without a crash.
> This is almost 50% of the work done without any issue. Before, a crash
> would happen within minutes, sometimes 1 hour, but not much more. The
> problem is, I didn't change anything to the filesystem, well, appart
> from the benign quota disable. So Qu's question about the qgroups
> errors in fsck made me wonder : if I activate quota again, it'll still
> continue to balance flawlessly, right ?
> 
> Well, it doesn't. I just ran btrfs quota enable on my filesystem, it
> completed successfully after some minutes (rescan -s said that no
> rescan was pending). Then less than 5 minutes later, the kernel
> crashed, at the same BUG_ON() than usually :
> 
> [60156.062082] BTRFS info (device dm-3): relocating block group
> 972839452672 flags 129
> [60185.203626] BTRFS info (device dm-3): found 1463 extents
> [60414.452890] {btrfs} in insert_inline_extent_backref, got owner <
> BTRFS_FIRST_FREE_OBJECTID
> [60414.452894] {btrfs} with bytenr=5197436141568 num_bytes=16384
> parent=5336636473344 root_objectid=3358 owner=1 offset=0 refs_to_add=1
> BTRFS_FIRST_FREE_OBJECTID=256
> [60414.452924] ------------[ cut here ]------------
> [60414.452928] kernel BUG at fs/btrfs/extent-tree.c:1837!
> 
> owner is=1 again at this point in the code (this is still kernel
> 4.3.0-rc1 with my added printks).
> 
> So I'll disable quota, again, and resume the balance. If I'm right, it
> should proceed without issue for 18 more hours !

Damn, wrong again. It just re-crashed without quota enabled :(
The fact that it went perfectly well for 17+ hours and crashed minutes 
after I reactivated quota might be by complete chance then ...

[ 5487.706499] {btrfs} in insert_inline_extent_backref, got owner < 
BTRFS_FIRST_FREE_OBJECTID
[ 5487.706504] {btrfs} with bytenr=6906661109760 num_bytes=16384 
parent=6905020874752 root_objectid=18446744073709551608 owner=1 offset=0 
refs_to_add=1 BTRFS_FIRST_FREE_OBJECTID=256
[ 5487.706536] ------------[ cut here ]------------
[ 5487.706539] kernel BUG at fs/btrfs/extent-tree.c:1837!

For reference, the crash I had earlier this morning was as follows :

[60414.452894] {btrfs} with bytenr=5197436141568 num_bytes=16384 
parent=5336636473344 root_objectid=3358 owner=1 offset=0 refs_to_add=1 
BTRFS_FIRST_FREE_OBJECTID=256

So, this is a completely different part of the filesystem.
The bug is always the same though, owner=1 where it shouldn't be < 256.

Balance cancelled.

To me, it sounds like some sort of race condition. But I'm out of ideas 
on what to test now.

-- 
Stéphane.