btrfs: avoid possible signal interruption of btrfs_drop_snapshot() on relocation tree
commit f3e3d9cc35
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.com>3
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
5.4-rM2-2.2.x-imx-squashed
parent
2f29a31f39
commit
e0e51f4fc4
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@ -5221,7 +5221,14 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
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goto out;
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}
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trans = btrfs_start_transaction(tree_root, 0);
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/*
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* Use join to avoid potential EINTR from transaction start. See
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* wait_reserve_ticket and the whole reservation callchain.
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*/
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if (for_reloc)
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trans = btrfs_join_transaction(tree_root);
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else
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trans = btrfs_start_transaction(tree_root, 0);
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if (IS_ERR(trans)) {
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err = PTR_ERR(trans);
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goto out_free;
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