RE: [BUG] kernel BUG at fs/btrfs/scrub.c:1956!, when scrubbing freshly converted ext4

[Zhao Lei <zhaolei@cn.fujitsu.com>]

Hi, Chris Murphy

Thanks for report this bug.

> -----Original Message-----
> From: linux-btrfs-owner@vger.kernel.org
> [mailto:linux-btrfs-owner@vger.kernel.org] On Behalf Of Chris Murphy
> Sent: Thursday, July 09, 2015 11:16 AM
> To: Btrfs BTRFS
> Subject: [BUG] kernel BUG at fs/btrfs/scrub.c:1956!, when scrubbing freshly
> converted ext4
> 
> OK yet another btrfs-convert bug. This one is easier to reproduce than the
> others, does not damage the file system (still mounts OK) but the oops is a hard
> panic. ssh and even the virt-manager console dies instantly, but virsh console
> captured a lot of call traces and are attached to the bug report.
> 
> Happens with 4.1 and 4.2rc1 kernels. The convert was done with btrfs-progs
> 4.1.
> 
> The reproduce steps are easy:
> 
> 1. ext4 with a bunch of files (Fedora 22 installation) plus a ~1.5GB ISO 2.
> btrfs-convert using defaults goes OK no error, btrfs check has no complaints 3.
> mount 4. btrfs scrub hard panic
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=101231
> 

I reproduced above bug too, with following command:
  mkfs.ext4 /dev/vdh
  btrfs-convert /dev/vdh
  mount /dev/vdh /mnt/tmp1
  btrfs scrub start -B /dev/vdh
  (panic)

The reason is:
1: In some case, metadata(leaf) created by btrfs-convert was split into 2 strips.
2: Then scrub bypassed part of leaf data, and left data caused panic in
  scrub_checksum_tree_block().

For above 1:
we can get following information after some simple operation.
a. mkfs.ext4 /dev/vdh
  btrfs-convert /dev/vdh
b. btrfs-debug-tree /dev/vdh
  we can see following item in extent tree:
  item 25 key (27054080 METADATA_ITEM 0) itemoff 15083 itemsize 33
  Its logical address is [27054080, 27070464)
  and acrossed 2 strips:
  [27000832, 27066368)
  [27066368, 27131904)
Qu Wenruo <quwenruo@cn.fujitsu.com> is fixing above problem.

For above 2:
scrub is trying to do a "bypass" in this case, but the result is "panic".
I'll fix it.

Thanks
Zhaolei

> 
> --
> Chris Murphy
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