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@xxxxxxxx>
Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
Thanks,
Josef
