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btrfs: avoid starting and committing empty transaction when flushing space

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When flushing space and we are in the COMMIT_TRANS state, we join a
transaction with btrfs_join_transaction() and then commit the returned
transaction. However btrfs_join_transaction() starts a new transaction if
there is none currently open, which is pointless since comitting a new,
empty transaction, doesn't achieve anything, it only wastes time, IO and
creates an unnecessary rotation of the backup roots.

So use btrfs_attach_transaction_barrier() to avoid starting a new
transaction. This also waits for any ongoing transaction that is
committing (state >= TRANS_STATE_COMMIT_DOING) to fully complete, and
therefore wait for all the extents that were pinned during the
transaction's lifetime to be unpinned.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Filipe Manana 2023-07-26 16:57:11 +01:00 committed by David Sterba
parent 2391245ac2
commit 2ee70ed19c
1 changed files with 10 additions and 1 deletions
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11
fs/btrfs/space-info.c
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@ -814,9 +814,18 @@ static void flush_space(struct btrfs_fs_info *fs_info,
break;
case COMMIT_TRANS:
ASSERT(current->journal_info == NULL);
trans = btrfs_join_transaction(root);
/*
* We don't want to start a new transaction, just attach to the
* current one or wait it fully commits in case its commit is
* happening at the moment. Note: we don't use a nostart join
* because that does not wait for a transaction to fully commit
* (only for it to be unblocked, state TRANS_STATE_UNBLOCKED).
*/
trans = btrfs_attach_transaction_barrier(root);
if (IS_ERR(trans)) {
ret = PTR_ERR(trans);
if (ret == -ENOENT)
ret = 0;
break;
}
ret = btrfs_commit_transaction(trans);