btrfs: introduce a FORCE_COMMIT_TRANS flush operation

Solely for preemptive flushing, we want to be able to force the
transaction commit without any of the ambiguity of
may_commit_transaction().  This is because may_commit_transaction()
checks tickets and such, and in preemptive flushing we already know
it'll be helpful, so use this to keep the code nice and clean and
straightforward.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[ add comment ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -2740,6 +2740,7 @@ enum btrfs_flush_state {
 	ALLOC_CHUNK_FORCE	=	8,
 	RUN_DELAYED_IPUTS	=	9,
 	COMMIT_TRANS		=	10,
+	FORCE_COMMIT_TRANS	=	11,
 };
 
 int btrfs_subvolume_reserve_metadata(struct btrfs_root *root,

fs/btrfs/space-info.c

@@ -140,6 +140,12 @@
  *     be freed, plus any delayed work we may not have gotten rid of in the case
  *     of metadata.
  *
+ *   FORCE_COMMIT_TRANS
+ *     For use by the preemptive flusher.  We use this to bypass the ticketing
+ *     checks in may_commit_transaction, as we have more information about the
+ *     overall state of the system and may want to commit the transaction ahead
+ *     of actual ENOSPC conditions.
+ *
  * OVERCOMMIT
  *
  *   Because we hold so many reservations for metadata we will allow you to
@@ -735,6 +741,14 @@ static void flush_space(struct btrfs_fs_info *fs_info,
 	case COMMIT_TRANS:
 		ret = may_commit_transaction(fs_info, space_info);
 		break;
+	case FORCE_COMMIT_TRANS:
+		trans = btrfs_join_transaction(root);
+		if (IS_ERR(trans)) {
+			ret = PTR_ERR(trans);
+			break;
+		}
+		ret = btrfs_commit_transaction(trans);
+		break;
 	default:
 		ret = -ENOSPC;
 		break;

include/trace/events/btrfs.h

@@ -99,7 +99,8 @@ struct btrfs_space_info;
 	EM( ALLOC_CHUNK,		"ALLOC_CHUNK")			\
 	EM( ALLOC_CHUNK_FORCE,		"ALLOC_CHUNK_FORCE")		\
 	EM( RUN_DELAYED_IPUTS,		"RUN_DELAYED_IPUTS")		\
-	EMe(COMMIT_TRANS,		"COMMIT_TRANS")
+	EM( COMMIT_TRANS,		"COMMIT_TRANS")			\
+	EMe(FORCE_COMMIT_TRANS,		"FORCE_COMMIT_TRANS")
 
 /*
  * First define the enums in the above macros to be exported to userspace via