Re: [syzbot] [btrfs?] kernel BUG in prepare_to_merge

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2023/8/1 19:39, Christoph Hellwig wrote:
> With misc-next and your debug patch I first ran into another assert:
>
> [  250.848976][T35903] assertion failed: 0, in fs/btrfs/relocation.c:2042
> [  250.849963][T35903] ------------[ cut here ]------------
> [  250.850472][T35903] kernel BUG at fs/btrfs/relocation.c:2042!
>
> and here is the output from your assert:
>
> [ 1378.272143][T189001] BTRFS error (device loop1): reloc tree mismatch, root 8 has no reloc root, expect reloc root key (-8, 132, 8) gen 17

Thanks a lot!

This indeed shows what I feared, on-disk corruption.

The root 8 is quota tree, which doesn't need to go through tree-reloc at
all.

The whole tree-relocation idea is for subvolume trees, which would do a
special snapshot for them, and then swap the highest tree nodes between
the tree reloc tree (the special snapshot) and the subvolume tree.

Thus for non-subvolume trees, relocation is done by just COWing the
involved tree blocks and call it a day.

This means we should never hit a reloc tree for non-subvolume trees, and
this looks like a on-disk format corruption.

Maybe I can reject those obviously incorrect reloc trees in tree-checker.

Thanks,
Qu

> [ 1378.274019][T189001] ------------[ cut here ]------------
> [ 1378.274540][T189001] BTRFS: Transaction aborted (error -117)
> [ 1378.277110][T189001] WARNING: CPU: 3 PID: 189001 at fs/btrfs/relocation.c:1946 prepare_to_merge+0x10e0/0x1460
>