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The radix-tree has been superseded by the xarray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 253bf57555e451 ("btrfs: turn
delayed_nodes_tree into an XArray") and fixes the problems with locking
and GFP flags 088aea3b97e0ae ("Revert "btrfs: turn delayed_nodes_tree
into an XArray"").

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index a1f6e8d3b546..17cfba9ee273 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3805,7 +3805,7 @@ cache_index:
 	 * 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,