From: Qu Wenruo wqu@suse.com
commit f3e3d9cc35252a70a2fd698762c9687718268ec6 upstream.
[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 any other error, same for the signal, and exit ASAP.
However for balance, especially for the clean_dirty_subvols() case, we're already doing cleanup works, getting -EINTR from btrfs_drop_snapshot() could cause a lot of unexpected problems.
From the mentioned forced read-only report, 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.
Since btrfs_join_transaction() won't get interrupted by signal, we can continue the cleanup.
CC: stable@vger.kernel.org # 5.4+ Reviewed-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Qu Wenruo wqu@suse.com Reviewed-by: David Sterba dsterba@suse.com3 Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/btrfs/extent-tree.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)
--- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -5313,7 +5313,14 @@ int btrfs_drop_snapshot(struct btrfs_roo goto out; }
- trans = btrfs_start_transaction(tree_root, 0); + /* + * Use join to avoid potential EINTR from transaction start. See + * wait_reserve_ticket and the whole reservation callchain. + */ + if (for_reloc) + trans = btrfs_join_transaction(tree_root); + else + trans = btrfs_start_transaction(tree_root, 0); if (IS_ERR(trans)) { err = PTR_ERR(trans); goto out_free;