Btrfs: ensure send always works on roots without orphans

Move the logic from the snapshot creation ioctl into send. This avoids
doing the transaction commit if send isn't used, and ensures that if
a crash/reboot happens after the transaction commit that created the
snapshot and before the transaction commit that switched the commit
root, send will not get a commit root that differs from the main root
(that has orphan items).
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ioctl.c

@@ -717,35 +717,6 @@ static int create_snapshot(struct btrfs_root *root, struct inode *dir,
 	if (ret)
 		goto fail;
 
-	/*
-	 * If orphan cleanup did remove any orphans, it means the tree was
-	 * modified and therefore the commit root is not the same as the
-	 * current root anymore. This is a problem, because send uses the
-	 * commit root and therefore can see inode items that don't exist
-	 * in the current root anymore, and for example make calls to
-	 * btrfs_iget, which will do tree lookups based on the current root
-	 * and not on the commit root. Those lookups will fail, returning a
-	 * -ESTALE error, and making send fail with that error. So make sure
-	 * a send does not see any orphans we have just removed, and that it
-	 * will see the same inodes regardless of whether a transaction
-	 * commit happened before it started (meaning that the commit root
-	 * will be the same as the current root) or not.
-	 */
-	if (readonly && pending_snapshot->snap->node !=
-	    pending_snapshot->snap->commit_root) {
-		trans = btrfs_join_transaction(pending_snapshot->snap);
-		if (IS_ERR(trans) && PTR_ERR(trans) != -ENOENT) {
-			ret = PTR_ERR(trans);
-			goto fail;
-		}
-		if (!IS_ERR(trans)) {
-			ret = btrfs_commit_transaction(trans,
-						       pending_snapshot->snap);
-			if (ret)
-				goto fail;
-		}
-	}
-
 	inode = btrfs_lookup_dentry(dentry->d_parent->d_inode, dentry);
 	if (IS_ERR(inode)) {
 		ret = PTR_ERR(inode);

fs/btrfs/send.c

@@ -5507,6 +5507,51 @@ static int send_subvol(struct send_ctx *sctx)
 	return ret;
 }
 
+/*
+ * If orphan cleanup did remove any orphans from a root, it means the tree
+ * was modified and therefore the commit root is not the same as the current
+ * root anymore. This is a problem, because send uses the commit root and
+ * therefore can see inode items that don't exist in the current root anymore,
+ * and for example make calls to btrfs_iget, which will do tree lookups based
+ * on the current root and not on the commit root. Those lookups will fail,
+ * returning a -ESTALE error, and making send fail with that error. So make
+ * sure a send does not see any orphans we have just removed, and that it will
+ * see the same inodes regardless of whether a transaction commit happened
+ * before it started (meaning that the commit root will be the same as the
+ * current root) or not.
+ */
+static int ensure_commit_roots_uptodate(struct send_ctx *sctx)
+{
+	int i;
+	struct btrfs_trans_handle *trans = NULL;
+
+again:
+	if (sctx->parent_root &&
+	    sctx->parent_root->node != sctx->parent_root->commit_root)
+		goto commit_trans;
+
+	for (i = 0; i < sctx->clone_roots_cnt; i++)
+		if (sctx->clone_roots[i].root->node !=
+		    sctx->clone_roots[i].root->commit_root)
+			goto commit_trans;
+
+	if (trans)
+		return btrfs_end_transaction(trans, sctx->send_root);
+
+	return 0;
+
+commit_trans:
+	/* Use any root, all fs roots will get their commit roots updated. */
+	if (!trans) {
+		trans = btrfs_join_transaction(sctx->send_root);
+		if (IS_ERR(trans))
+			return PTR_ERR(trans);
+		goto again;
+	}
+
+	return btrfs_commit_transaction(trans, sctx->send_root);
+}
+
 static void btrfs_root_dec_send_in_progress(struct btrfs_root* root)
 {
 	spin_lock(&root->root_item_lock);
@@ -5728,6 +5773,10 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
 			NULL);
 	sort_clone_roots = 1;
 
+	ret = ensure_commit_roots_uptodate(sctx);
+	if (ret)
+		goto out;
+
 	current->journal_info = BTRFS_SEND_TRANS_STUB;
 	ret = send_subvol(sctx);
 	current->journal_info = NULL;