Re: [PATCH 1/2] btrfs: avoid possible signal interruption for btrfs_drop_snapshot() on relocation tree

[Josef Bacik <josef@toxicpanda.com>]

On 7/8/20 6:00 AM, Qu Wenruo wrote:
> [BUG]
> There is a bug report about bad signal timing could lead to read-only
> fs during balance:
> 
>    BTRFS info (device xvdb): balance: start -d -m -s
>    BTRFS info (device xvdb): relocating block group 73001861120 flags metadata
>    BTRFS info (device xvdb): found 12236 extents, stage: move data extents
>    BTRFS info (device xvdb): relocating block group 71928119296 flags data
>    BTRFS info (device xvdb): found 3 extents, stage: move data extents
>    BTRFS info (device xvdb): found 3 extents, stage: update data pointers
>    BTRFS info (device xvdb): relocating block group 60922265600 flags metadata
>    BTRFS: error (device xvdb) in btrfs_drop_snapshot:5505: errno=-4 unknown
>    BTRFS info (device xvdb): forced readonly
>    BTRFS info (device xvdb): balance: ended with status: -4
> 
> [CAUSE]
> The direct cause is the -EINTR from the following call chain when a
> fatal signal is pending:
> 
>   relocate_block_group()
>   |- clean_dirty_subvols()
>      |- btrfs_drop_snapshot()
>         |- btrfs_start_transaction()
>            |- btrfs_delayed_refs_rsv_refill()
>               |- btrfs_reserve_metadata_bytes()
>                  |- __reserve_metadata_bytes()
>                     |- wait_reserve_ticket()
>                        |- prepare_to_wait_event();
>                        |- ticket->error = -EINTR;
> 
> Normally this behavior is fine for most btrfs_start_transaction()
> callers, as they need to catch the fatal signal and exit asap.
> 
> However to balance, especially for the clean_dirty_subvols() case, we're
> already doing cleanup works, such -EINTR from btrfs_drop_snapshot()
> could cause a lot of unexpected problems.
> 
>  From the mentioned forced read-only, to later balance error due to half
> dropped reloc trees.
> 
> [FIX]
> Fix this problem by using btrfs_join_transaction() if
> btrfs_drop_snapshot() is called from relocation context.
> 
> As btrfs_join_transaction() won't wait full tickets, it won't get
> interrupted from signal.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef