Re: 5.6-5.10 balance regression?

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

On 2020/12/29 上午3:58, Stéphane Lesimple wrote:
>> I know it fails in relocate_block_group(), which returns -2, I'm currently
>> adding a couple printk's here and there to try to pinpoint that better.
>
> Okay, so btrfs_relocate_block_group() starts with stage MOVE_DATA_EXTENTS, which
> completes successfully, as relocate_block_group() returns 0:
>
> BTRFS info (device <unknown>): relocate_block_group: prepare_to_realocate = 0
> BTRFS info (device <unknown>): relocate_block_group loop: progress = 1, btrfs_start_transaction = ok
> [...]
> BTRFS info (device <unknown>): relocate_block_group loop: progress = 168, btrfs_start_transaction = ok
> BTRFS info (device <unknown>): relocate_block_group: returning err = 0
> BTRFS info (device dm-10): stage = move data extents, relocate_block_group = 0
> BTRFS info (device dm-10): found 167 extents, stage: move data extents
>
> Then it proceeds to the UPDATE_DATA_PTRS stage and calls relocate_block_group()
> again. This time it'll fail at the 92th iteration of the loop:
>
> BTRFS info (device <unknown>): relocate_block_group loop: progress = 92, btrfs_start_transaction = ok
> BTRFS info (device <unknown>): relocate_block_group loop: extents_found = 92, item_size(53) >= sizeof(*ei)(24), flags = 1, ret = 0
> BTRFS info (device <unknown>): add_data_references: btrfs_find_all_leafs = 0
> BTRFS info (device <unknown>): add_data_references loop: read_tree_block ok
> BTRFS info (device <unknown>): add_data_references loop: delete_v1_space_cache = -2

Damn it, if we find no v1 space cache for the block group, it means
we're fine to continue...

> BTRFS info (device <unknown>): relocate_block_group loop: add_data_references = -2
>
> Then the -ENOENT goes all the way up the call stack and aborts the balance.
>
> So it fails in delete_v1_space_cache(), though it is worth noting that the
> FS we're talking about is actually using space_cache v2.

Space cache v2, no wonder no v1 space cache.

>
> Does it help? Shall I dig deeper?

You're already at the point!

Mind me to craft a fix with your signed-off-by?

Thanks,
Qu

>
> Regards,
>
> Stéphane.
>