Re: [PATCH] btrfs: reloc: clear DEAD_RELOC_TREE bit for orphan roots to prevent runaway balance

Greg KH
2020-07-24T09:47:19Z

On Fri, Jul 24, 2020 at 09:46:40AM +0800, Qu Wenruo wrote:
> commit 1dae7e0e58b484eaa43d530f211098fdeeb0f404 upstream.
>
> [BUG]
> There are several reported runaway balance, that balance is flooding the
> log with "found X extents" where the X never changes.
>
> [CAUSE]
> Commit d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after
> merge_reloc_roots") introduced BTRFS_ROOT_DEAD_RELOC_TREE bit to
> indicate that one subvolume has finished its tree blocks swap with its
> reloc tree.
>
> However if balance is canceled or hits ENOSPC halfway, we didn't clear
> the BTRFS_ROOT_DEAD_RELOC_TREE bit, leaving that bit hanging forever
> until unmount.
>
> Any subvolume root with that bit, would cause backref cache to skip this
> tree block, as it has finished its tree block swap. This would cause
> all tree blocks of that root be ignored by balance, leading to runaway
> balance.
>
> [FIX]
> Fix the problem by also clearing the BTRFS_ROOT_DEAD_RELOC_TREE bit for
> the original subvolume of orphan reloc root.
>
> Add an umount check for the stale bit still set.
>
> Fixes: d2311e698578 ("btrfs: relocation: Delay reloc tree deletion after merge_reloc_roots")
> Cc: <stable@vger.kernel.org> # 5.7.x
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
> fs/btrfs/disk-io.c | 1 +
> fs/btrfs/relocation.c | 2 ++
> 2 files changed, 3 insertions(+)

Thanks for the backports, all now queued up.

greg k-h