Re: [PATCH 2/6] Btrfs: fix crash caused by block group removal

[Josef Bacik <jbacik@fb.com>]

On 11/26/2014 10:28 AM, Filipe Manana wrote:
> If we remove a block group (because it became empty), we might have left
> a caching_ctl structure in fs_info->caching_block_groups that points to
> the block group and is accessed at transaction commit time. This results
> in accessing an invalid or incorrect block group. This issue became visible
> after Josef's patch "Btrfs: remove empty block groups automatically".
>
> So if the block group is removed make sure we don't leave a dangling
> caching_ctl in caching_block_groups.
>
> Sample crash trace:
>
> [58380.439449] BUG: unable to handle kernel paging request at ffff8801446eaeb8
> [58380.439707] IP: [<ffffffffa03f6d05>] block_group_cache_done.isra.21+0xc/0x1c [btrfs]
> [58380.440879] PGD 1acb067 PUD 23f5ff067 PMD 23f5db067 PTE 80000001446ea060
> [58380.441220] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
> [58380.441486] Modules linked in: btrfs crc32c_generic xor raid6_pq nfsd auth_rpcgss oid_registry nfs_acl nfs lockd fscache sunrpc loop psmouse processor i2c_piix4 parport_pc parport pcspkr serio_raw evdev i2ccore thermal_sys microcode button ext4 crc16 jbd2 mbcache sr_mod cdrom ata_generic sg sd_mod crc_t10dif crct10dif_generic crct10dif_common virtio_scsi floppy ata_piix e1000 libata virtio_pci scsi_mod virtio_ring virtio [last unloaded: btrfs]
> [58380.443238] CPU: 3 PID: 25728 Comm: btrfs-transacti Tainted: G        W      3.17.0-rc5-btrfs-next-1+ #1
> [58380.443238] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
> [58380.443238] task: ffff88013ac82090 ti: ffff88013896c000 task.ti: ffff88013896c000
> [58380.443238] RIP: 0010:[<ffffffffa03f6d05>]  [<ffffffffa03f6d05>] block_group_cache_done.isra.21+0xc/0x1c [btrfs]
> [58380.443238] RSP: 0018:ffff88013896fdd8  EFLAGS: 00010283
> [58380.443238] RAX: ffff880222cae850 RBX: ffff880119ba74c0 RCX: 0000000000000000
> [58380.443238] RDX: 0000000000000000 RSI: ffff880185e16800 RDI: ffff8801446eaeb8
> [58380.443238] RBP: ffff88013896fdd8 R08: ffff8801a9ca9fa8 R09: ffff88013896fc60
> [58380.443238] R10: ffff88013896fd28 R11: 0000000000000000 R12: ffff880222cae000
> [58380.443238] R13: ffff880222cae850 R14: ffff880222cae6b0 R15: ffff8801446eae00
> [58380.443238] FS:  0000000000000000(0000) GS:ffff88023ed80000(0000) knlGS:0000000000000000
> [58380.443238] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [58380.443238] CR2: ffff8801446eaeb8 CR3: 0000000001811000 CR4: 00000000000006e0
> [58380.443238] Stack:
> [58380.443238]  ffff88013896fe18 ffffffffa03fe2d5 ffff880222cae850 ffff880185e16800
> [58380.443238]  ffff88000dc41c20 0000000000000000 ffff8801a9ca9f00 0000000000000000
> [58380.443238]  ffff88013896fe80 ffffffffa040fbcf ffff88018b0dcdb0 ffff88013ac82090
> [58380.443238] Call Trace:
> [58380.443238]  [<ffffffffa03fe2d5>] btrfs_prepare_extent_commit+0x5a/0xd7 [btrfs]
> [58380.443238]  [<ffffffffa040fbcf>] btrfs_commit_transaction+0x45c/0x882 [btrfs]
> [58380.443238]  [<ffffffffa040c058>] transaction_kthread+0xf2/0x1a4 [btrfs]
> [58380.443238]  [<ffffffffa040bf66>] ? btrfs_cleanup_transaction+0x3d8/0x3d8 [btrfs]
> [58380.443238]  [<ffffffff8105966b>] kthread+0xb7/0xbf
> [58380.443238]  [<ffffffff810595b4>] ? __kthread_parkme+0x67/0x67
> [58380.443238]  [<ffffffff813ebeac>] ret_from_fork+0x7c/0xb0
> [58380.443238]  [<ffffffff810595b4>] ? __kthread_parkme+0x67/0x67
>
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>   fs/btrfs/ctree.h       |  1 +
>   fs/btrfs/extent-tree.c | 27 +++++++++++++++++++++++++++
>   2 files changed, 28 insertions(+)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index d3ccd09..7f40a65 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1277,6 +1277,7 @@ struct btrfs_block_group_cache {
>   	unsigned int ro:1;
>   	unsigned int dirty:1;
>   	unsigned int iref:1;
> +	unsigned int has_caching_ctl:1;
>

Don't do this, just unconditionally call get_caching_control in 
btrfs_remove_block_group, then if we get something we can do stuff, 
otherwise we can just continue.  Thanks,

Josef