Re: [PATCH] btrfs: reloc: clear DEAD_RELOC_TREE bit for orphan roots -- for 5.7 to prevent runaway balance

[David Sterba <dsterba@suse.cz>]

Qu,

you need to make the target version visible in the subject. The manual
submissions to stable are processed by humans, while the CC: tag is for
semi-automatic collection of patches that get merged to master.

If you send 3 patches with same subjects it could be ignored as resends
or it's required to read all the changelogs.

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(+)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index f71e4dbe1d8a..f00e64fee5dd 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -1998,6 +1998,7 @@ void btrfs_put_root(struct btrfs_root *root)
>  
>  	if (refcount_dec_and_test(&root->refs)) {
>  		WARN_ON(!RB_EMPTY_ROOT(&root->inode_tree));
> +		WARN_ON(test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state));
>  		if (root->anon_dev)
>  			free_anon_bdev(root->anon_dev);
>  		btrfs_drew_lock_destroy(&root->snapshot_lock);
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 157452a5e110..f67d736c27a1 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -2642,6 +2642,8 @@ void merge_reloc_roots(struct reloc_control *rc)
>  					root->reloc_root = NULL;
>  					btrfs_put_root(reloc_root);
>  				}
> +				clear_bit(BTRFS_ROOT_DEAD_RELOC_TREE,
> +					  &root->state);
>  				btrfs_put_root(root);
>  			}
>  
> -- 
> 2.27.0