btrfs: preallocate inodes xarray entry to avoid transaction abort

When creating a new inode, at btrfs_create_new_inode(), one of the very
last steps is to add the inode to the root's inodes xarray. This often
requires allocating memory which may fail (even though xarrays have a
dedicated kmem_cache which make it less likely to fail), and at that point
we are forced to abort the current transaction (as some, but not all, of
the inode metadata was added to its subvolume btree).

To avoid a transaction abort, preallocate memory for the xarray early at
btrfs_create_new_inode(), so that if we fail we don't need to abort the
transaction and the insertion into the xarray is guaranteed to succeed.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -5493,7 +5493,7 @@ static int fixup_tree_root_location(struct btrfs_fs_info *fs_info,
 	return err;
 }
 
-static int btrfs_add_inode_to_root(struct btrfs_inode *inode)
+static int btrfs_add_inode_to_root(struct btrfs_inode *inode, bool prealloc)
 {
 	struct btrfs_root *root = inode->root;
 	struct btrfs_inode *existing;
@@ -5503,9 +5503,11 @@ static int btrfs_add_inode_to_root(struct btrfs_inode *inode)
 	if (inode_unhashed(&inode->vfs_inode))
 		return 0;
 
-	ret = xa_reserve(&root->inodes, ino, GFP_NOFS);
-	if (ret)
-		return ret;
+	if (prealloc) {
+		ret = xa_reserve(&root->inodes, ino, GFP_NOFS);
+		if (ret)
+			return ret;
+	}
 
 	spin_lock(&root->inode_lock);
 	existing = xa_store(&root->inodes, ino, inode, GFP_ATOMIC);
@@ -5606,7 +5608,7 @@ struct inode *btrfs_iget_path(struct super_block *s, u64 ino,
 
 		ret = btrfs_read_locked_inode(inode, path);
 		if (!ret) {
-			ret = btrfs_add_inode_to_root(BTRFS_I(inode));
+			ret = btrfs_add_inode_to_root(BTRFS_I(inode), true);
 			if (ret) {
 				iget_failed(inode);
 				inode = ERR_PTR(ret);
@@ -6237,6 +6239,7 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 	struct btrfs_item_batch batch;
 	unsigned long ptr;
 	int ret;
+	bool xa_reserved = false;
 
 	path = btrfs_alloc_path();
 	if (!path)
@@ -6251,6 +6254,11 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 		goto out;
 	inode->i_ino = objectid;
 
+	ret = xa_reserve(&root->inodes, objectid, GFP_NOFS);
+	if (ret)
+		goto out;
+	xa_reserved = true;
+
 	if (args->orphan) {
 		/*
 		 * O_TMPFILE, set link count to 0, so that after this point, we
@@ -6424,8 +6432,9 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 		}
 	}
 
-	ret = btrfs_add_inode_to_root(BTRFS_I(inode));
-	if (ret) {
+	ret = btrfs_add_inode_to_root(BTRFS_I(inode), false);
+	if (WARN_ON(ret)) {
+		/* Shouldn't happen, we used xa_reserve() before. */
 		btrfs_abort_transaction(trans, ret);
 		goto discard;
 	}
@@ -6456,6 +6465,9 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 	ihold(inode);
 	discard_new_inode(inode);
 out:
+	if (xa_reserved)
+		xa_release(&root->inodes, objectid);
+
 	btrfs_free_path(path);
 	return ret;
 }