If you create a seed device with btrfstune and then try to add a small (32MB) device to it, the kernel will oops and the attempt to add the device will fail.
I have attached a transcript of the shell session that demonstrates this. (This was generated by piping UML to tee and then cleaning up the file. I removed some control characters, but the backtrace is still intact.) This bug os triggered by a 32MB device, but not a 64MB one. This bug is still present with the rc6 kernel. Note that, in earlier kernels, the attempt to remount read-write would succeed, but process attempting to access the filesystem would hang forever, resisting even SIGKILL.
raid6: int32x1 695 MB/s raid6: int32x2 678 MB/s raid6: int32x4 689 MB/s raid6: int32x8 413 MB/s raid6: using algorithm int32x1 (695 MB/s) raid6: using intx1 recovery algorithm xor: measuring software checksum speed 8regs : 4674.000 MB/sec 8regs_prefetch: 4226.000 MB/sec 32regs : 2824.400 MB/sec 32regs_prefetch: 4100.800 MB/sec xor: using function: 8regs (4674.000 MB/sec) bio: create slab <bio-1> at 1 Btrfs loaded btrfs: device fsid 05e9ab31-1ae3-4195-a5dd-7ce0943236db devid 1 transid 451 /dev/ubda btrfs: disk space caching is enabled root@(none):~# truncate st -s 64M root@(none):~# mkfs.btrfs st WARNING! - Btrfs v0.20-rc1 IS EXPERIMENTAL WARNING! - see http://btrfs.wiki.kernel.org before using SMALL VOLUME: forcing mixed metadata/data groups Created a data/metadata chunk of size 6684672 fs created label (null) on st nodesize 4096 leafsize 4096 sectorsize 4096 size 64.00MB Btrfs v0.20-rc1 root@(none):~# btrfs -S 1 st root@(none):~# losetup -fv st Loop device is /dev/loop0 root@(none):~# mount /dev/loop0 /mnt EXT4-fs (loop0): VFS: Can't find ext4 filesystem EXT4-fs (loop0): VFS: Can't find ext4 filesystem EXT4-fs (loop0): VFS: Can't find ext4 filesystem btrfs: device fsid 63184a6c-3df9-47b4-bf7a-5f68ebb24761 devid 1 transid 5 /dev/loop0 mount: block device /dev/loop0 is write-protected, mounting read-only EXT4-fs (loop0): VFS: Can't find ext4 filesystem EXT4-fs (loop0): VFS: Can't find ext4 filesystem EXT4-fs (loop0): VFS: Can't find ext4 filesystem btrfs: device fsid 63184a6c-3df9-47b4-bf7a-5f68ebb24761 devid 1 transid 5 /dev/loop0 btrfs: disk space caching is enabled root@(none):~# truncate st0 -s 32M root@(none):~# losetup-fv st0 Loop device is /dev/loop1 root@(none):~# btrfs device add /dev/loop1 /mnt SMALL VOLUME: forcing mixed metadata/data groups btrfs: relocating block group 0 flags 2 ------------[ cut here ]------------ WARNING: CPU: 0 PID: 343 at fs/btrfs/volumes.c:2554 btrfs_init_new_device+0xc71/0xd4b [btrfs]() Modules linked in: btrfs lzo_compress zlib_deflate xor raid6_pq libcrc32c CPU: 0 PID: 343 Comm: btrfs Not tainted 3.12.0-rc5 #1 0ff57c14 08076736 082789fb 00000000 00000157 111e068a 000009fa 1119de3b 1119de3b 00000000 0ff48c00 003fffff 00000000 1049f738 1119de3b 00000009 00000000 00000100 00000000 000000e4 00000000 10505601 0ff82c00 00000001 0ff57c18: [<08076736>] warn_slowpath_common+0x78/0x99 0ff57c30: [<1119de3b>] btrfs_init_new_device+0xc71/0xd4b [btrfs] 0ff57c34: [<1119de3b>] btrfs_init_new_device+0xc71/0xd4b [btrfs] 0ff57c4c: [<1119de3b>] btrfs_init_new_device+0xc71/0xd4b [btrfs] 0ff57cec: [<111a43d6>] btrfs_ioctl+0x6cf/0x2a8c [btrfs] 0ff57d0c: [<08095294>] enqueue_task_fair+0xe3/0x195 0ff57d28: [<08073f2e>] os_set_thread_area+0x25/0x36 0ff57d3c: [<08073096>] do_set_thread_area+0x1a/0x3e 0ff57d54: [<080731aa>] arch_switch_tls+0x85/0xb2 0ff57d70: [<08071e52>] arch_switch_to+0xf/0x40 0ff57d78: [<080924fa>] finish_task_switch.isra.60+0x1b/0x54 0ff57db0: [<0822989c>] schedule+0x57/0x59 0ff57dbc: [<0822851f>] schedule_timeout+0x17/0xfd 0ff57ddc: [<08092325>] __wake_up+0x29/0x31 0ff57dec: [<111a3d07>] btrfs_ioctl+0x0/0x2a8c [btrfs] 0ff57dfc: [<080e8245>] vfs_ioctl+0x26/0x32 0ff57e00: [<0822942b>] wait_for_common+0xfc/0x106 0ff57e04: [<080e8245>] vfs_ioctl+0x26/0x32 0ff57e18: [<080e8dfb>] do_vfs_ioctl+0x526/0x603 0ff57e34: [<081ad651>] loop_switch.isra.22+0x7e/0x8f 0ff57e54: [<080ee72b>] iput+0x5d/0x113 0ff57e64: [<081adaaa>] lo_release+0x49/0x59 0ff57e74: [<0810222e>] __blkdev_put+0xfe/0x116 0ff57e88: [<080e8f21>] SyS_ioctl+0x49/0x68 0ff57eb4: [<08060c9a>] handle_syscall+0x6d/0x7f 0ff57ef4: [<08070a75>] userspace+0x3c9/0x49c 0ff57f88: [<08060e92>] do_op_one_page+0xfb/0x17f 0ff57f9c: [<08073f2e>] os_set_thread_area+0x25/0x36 0ff57fb0: [<08073096>] do_set_thread_area+0x1a/0x3e 0ff57fbc: [<08089500>] __task_pid_nr_ns+0x51/0x5b 0ff57fc8: [<080731aa>] arch_switch_tls+0x85/0xb2 0ff57ff0: [<0805e648>] fork_handler+0x61/0x65 ---[ end trace 729c0d24bd03140c ]--- BTRFS error (device loop0) in btrfs_init_new_device:2124: errno=-28 No space left (Failed to relocate sys chunks after device initialization. This can be fixed using the "btrfs balance" command.) BTRFS info (device loop0): forced readonly ERROR: error adding the device '/dev/loop1' - Read-only file system root@(none):~# mount -o remount,rw /mnt btrfs: disk space caching is enabled BTRFS error (device loop0): Remounting read-write after error is not allowed mount: /mnt not mounted or bad option root@(none):~# uname -a Linux (none) 3.12.0-rc5 #1 Sun Oct 13 20:08:45 CDT 2013 i686 GNU/Linux root@(none):~# btrfs version Btrfs v0.20-rc1
