btrfs: Deprecate BTRFS_SUBVOL_CREATE_ASYNC flag

Support for asynchronous snapshot creation was originally added in
72fd032 ("Btrfs: add SNAP_CREATE_ASYNC ioctl") to cater for
ceph's backend needs. However, since Ceph has deprecated support for
btrfs there is no longer need for that support in btrfs. Additionally,
this was never supported by btrfs-progs, the official userspace tools.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -1841,8 +1841,15 @@ static noinline int btrfs_ioctl_snap_create_v2(struct file *file,
 		goto free_args;
 	}
 
-	if (vol_args->flags & BTRFS_SUBVOL_CREATE_ASYNC)
+	if (vol_args->flags & BTRFS_SUBVOL_CREATE_ASYNC) {
+		struct inode *inode = file_inode(file);
+		struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
+
+		btrfs_warn(fs_info,
+"SNAP_CREATE_V2 ioctl with CREATE_ASYNC is deprecated and will be removed in kernel 5.7");
+
 		ptr = &transid;
+	}
 	if (vol_args->flags & BTRFS_SUBVOL_RDONLY)
 		readonly = true;
 	if (vol_args->flags & BTRFS_SUBVOL_QGROUP_INHERIT) {
@@ -4191,6 +4198,9 @@ static noinline long btrfs_ioctl_start_sync(struct btrfs_root *root,
 	u64 transid;
 	int ret;
 
+	btrfs_warn(root->fs_info,
+	"START_SYNC ioctl is deprecated and will be removed in kernel 5.7");
+
 	trans = btrfs_attach_transaction_barrier(root);
 	if (IS_ERR(trans)) {
 		if (PTR_ERR(trans) != -ENOENT)
@@ -4218,6 +4228,9 @@ static noinline long btrfs_ioctl_wait_sync(struct btrfs_fs_info *fs_info,
 {
 	u64 transid;
 
+	btrfs_warn(fs_info,
+		"WAIT_SYNC ioctl is deprecated and will be removed in kernel 5.7");
+
 	if (argp) {
 		if (copy_from_user(&transid, argp, sizeof(transid)))
 			return -EFAULT;