btrfs: switch btrfs_root::delayed_nodes_tree to xarray from radix-tree

The radix-tree has been superseded by the xarray
(https://lwn.net/Articles/745073), this patch converts the
btrfs_root::delayed_nodes, the APIs are used in a simple way.

First idea is to do xa_insert() but this would require GFP_ATOMIC
allocation which we want to avoid if possible. The preload mechanism of
radix-tree can be emulated within the xarray API.

- xa_reserve() with GFP_NOFS outside of the lock, the reserved entry
  is inserted atomically at most once

- xa_store() under a lock, in case something races in we can detect that
  and xa_load() returns a valid pointer

All uses of xa_load() must check for a valid pointer in case they manage
to get between the xa_reserve() and xa_store(), this is handled in
btrfs_get_delayed_node().

Otherwise the functionality is equivalent, xarray implements the
radix-tree and there should be no performance difference.

The patch continues the efforts started in 253bf575 ("btrfs: turn
delayed_nodes_tree into an XArray") and fixes the problems with locking
and GFP flags 088aea3b ("Revert "btrfs: turn delayed_nodes_tree
into an XArray"").
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -227,10 +227,10 @@ struct btrfs_root {
 	struct rb_root inode_tree;
 
 	/*
-	 * radix tree that keeps track of delayed nodes of every inode,
-	 * protected by inode_lock
+	 * Xarray that keeps track of delayed nodes of every inode, protected
+	 * by @inode_lock.
 	 */
-	struct radix_tree_root delayed_nodes_tree;
+	struct xarray delayed_nodes;
 	/*
 	 * right now this just gets used so that a root has its own devid
 	 * for stat.  It may be used for more later

fs/btrfs/delayed-inode.c

@@ -71,7 +71,7 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
 	}
 
 	spin_lock(&root->inode_lock);
-	node = radix_tree_lookup(&root->delayed_nodes_tree, ino);
+	node = xa_load(&root->delayed_nodes, ino);
 
 	if (node) {
 		if (btrfs_inode->delayed_node) {
@@ -83,9 +83,9 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
 
 		/*
 		 * It's possible that we're racing into the middle of removing
-		 * this node from the radix tree.  In this case, the refcount
+		 * this node from the xarray.  In this case, the refcount
 		 * was zero and it should never go back to one.  Just return
-		 * NULL like it was never in the radix at all; our release
+		 * NULL like it was never in the xarray at all; our release
 		 * function is in the process of removing it.
 		 *
 		 * Some implementations of refcount_inc refuse to bump the
@@ -93,7 +93,7 @@ static struct btrfs_delayed_node *btrfs_get_delayed_node(
 		 * here, refcount_inc() may decide to just WARN_ONCE() instead
 		 * of actually bumping the refcount.
 		 *
-		 * If this node is properly in the radix, we want to bump the
+		 * If this node is properly in the xarray, we want to bump the
 		 * refcount twice, once for the inode and once for this get
 		 * operation.
 		 */
@@ -120,6 +120,7 @@ static struct btrfs_delayed_node *btrfs_get_or_create_delayed_node(
 	struct btrfs_root *root = btrfs_inode->root;
 	u64 ino = btrfs_ino(btrfs_inode);
 	int ret;
+	void *ptr;
 
 again:
 	node = btrfs_get_delayed_node(btrfs_inode);
@@ -131,26 +132,30 @@ static struct btrfs_delayed_node *btrfs_get_or_create_delayed_node(
 		return ERR_PTR(-ENOMEM);
 	btrfs_init_delayed_node(node, root, ino);
 
-	/* cached in the btrfs inode and can be accessed */
+	/* Cached in the inode and can be accessed. */
 	refcount_set(&node->refs, 2);
 
-	ret = radix_tree_preload(GFP_NOFS);
-	if (ret) {
+	/* Allocate and reserve the slot, from now it can return a NULL from xa_load(). */
+	ret = xa_reserve(&root->delayed_nodes, ino, GFP_NOFS);
+	if (ret == -ENOMEM) {
 		kmem_cache_free(delayed_node_cache, node);
-		return ERR_PTR(ret);
+		return ERR_PTR(-ENOMEM);
 	}
-
 	spin_lock(&root->inode_lock);
-	ret = radix_tree_insert(&root->delayed_nodes_tree, ino, node);
-	if (ret == -EEXIST) {
+	ptr = xa_load(&root->delayed_nodes, ino);
+	if (ptr) {
+		/* Somebody inserted it, go back and read it. */
 		spin_unlock(&root->inode_lock);
 		kmem_cache_free(delayed_node_cache, node);
-		radix_tree_preload_end();
+		node = NULL;
 		goto again;
 	}
+	ptr = xa_store(&root->delayed_nodes, ino, node, GFP_ATOMIC);
+	ASSERT(xa_err(ptr) != -EINVAL);
+	ASSERT(xa_err(ptr) != -ENOMEM);
+	ASSERT(ptr == NULL);
 	btrfs_inode->delayed_node = node;
 	spin_unlock(&root->inode_lock);
-	radix_tree_preload_end();
 
 	return node;
 }
@@ -269,8 +274,7 @@ static void __btrfs_release_delayed_node(
 		 * back up.  We can delete it now.
 		 */
 		ASSERT(refcount_read(&delayed_node->refs) == 0);
-		radix_tree_delete(&root->delayed_nodes_tree,
-				  delayed_node->inode_id);
+		xa_erase(&root->delayed_nodes, delayed_node->inode_id);
 		spin_unlock(&root->inode_lock);
 		kmem_cache_free(delayed_node_cache, delayed_node);
 	}
@@ -2038,34 +2042,36 @@ void btrfs_kill_delayed_inode_items(struct btrfs_inode *inode)
 
 void btrfs_kill_all_delayed_nodes(struct btrfs_root *root)
 {
-	u64 inode_id = 0;
+	unsigned long index = 0;
 	struct btrfs_delayed_node *delayed_nodes[8];
-	int i, n;
 
 	while (1) {
+		struct btrfs_delayed_node *node;
+		int count;
+
 		spin_lock(&root->inode_lock);
-		n = radix_tree_gang_lookup(&root->delayed_nodes_tree,
-					   (void **)delayed_nodes, inode_id,
-					   ARRAY_SIZE(delayed_nodes));
-		if (!n) {
+		if (xa_empty(&root->delayed_nodes)) {
 			spin_unlock(&root->inode_lock);
-			break;
+			return;
 		}
 
-		inode_id = delayed_nodes[n - 1]->inode_id + 1;
-		for (i = 0; i < n; i++) {
+		count = 0;
+		xa_for_each_start(&root->delayed_nodes, index, node, index) {
 			/*
 			 * Don't increase refs in case the node is dead and
 			 * about to be removed from the tree in the loop below
 			 */
-			if (!refcount_inc_not_zero(&delayed_nodes[i]->refs))
-				delayed_nodes[i] = NULL;
+			if (refcount_inc_not_zero(&node->refs)) {
+				delayed_nodes[count] = node;
+				count++;
+			}
+			if (count >= ARRAY_SIZE(delayed_nodes))
+				break;
 		}
 		spin_unlock(&root->inode_lock);
+		index++;
 
-		for (i = 0; i < n; i++) {
-			if (!delayed_nodes[i])
-				continue;
+		for (int i = 0; i < count; i++) {
 			__btrfs_kill_delayed_node(delayed_nodes[i]);
 			btrfs_release_delayed_node(delayed_nodes[i]);
 		}

fs/btrfs/disk-io.c

@@ -655,7 +655,8 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
 	root->nr_delalloc_inodes = 0;
 	root->nr_ordered_extents = 0;
 	root->inode_tree = RB_ROOT;
-	INIT_RADIX_TREE(&root->delayed_nodes_tree, GFP_ATOMIC);
+	/* GFP flags are compatible with XA_FLAGS_*. */
+	xa_init_flags(&root->delayed_nodes, GFP_ATOMIC);
 
 	btrfs_init_root_block_rsv(root);
 

fs/btrfs/inode.c

@@ -3805,7 +3805,7 @@ static int btrfs_read_locked_inode(struct inode *inode,
 	 * cache.
 	 *
 	 * This is required for both inode re-read from disk and delayed inode
-	 * in delayed_nodes_tree.
+	 * in the delayed_nodes xarray.
 	 */
 	if (BTRFS_I(inode)->last_trans == btrfs_get_fs_generation(fs_info))
 		set_bit(BTRFS_INODE_NEEDS_FULL_SYNC,