btrfs: use a regular rb_root instead of cached rb_root for extent_map_tree

We are currently using a cached rb_root (struct rb_root_cached) for the
rb root of struct extent_map_tree. This doesn't offer much of an advantage
here because:

1) It's only advantage over the regular rb_root is that it caches a
   pointer to the left most node (first node), so a call to
   rb_first_cached() doesn't have to chase pointers until it reaches
   the left most node;

2) We only have two scenarios that access left most node with
   rb_first_cached():

      When dropping all extent maps from an inode, during inode eviction;

      When iterating over extent maps during the extent map shrinker;

3) In both cases we keep removing extent maps, which causes deletion of
   the left most node so rb_erase_cached() has to call rb_next() to find
   out what's the next left most node and assign it to
   struct rb_root_cached::rb_leftmost;

4) We can do that ourselves in those two uses cases and stop using a
   rb_root_cached rb tree and use instead a regular rb_root rb tree.

   This reduces the size of struct extent_map_tree by 8 bytes and, since
   this structure is embedded in struct btrfs_inode, it also reduces the
   size of that structure by 8 bytes.

   So on a 64 bits platform the size of btrfs_inode is reduced from 1032
   bytes down to 1024 bytes.

   This means we will be able to have 4 inodes per 4K page instead of 3.

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

1 files changed, 3 insertions, 3 deletions

diff --git a/fs/btrfs/tests/extent-map-tests.c b/fs/btrfs/tests/extent-map-tests.c
index 075e6930acda..c511a1297956 100644
--- a/fs/btrfs/tests/extent-map-tests.c
+++ b/fs/btrfs/tests/extent-map-tests.c
@@ -19,8 +19,8 @@ static int free_extent_map_tree(struct btrfs_inode *inode)
 	int ret = 0;
 
 	write_lock(&em_tree->lock);
-	while (!RB_EMPTY_ROOT(&em_tree->root.rb_root)) {
-		node = rb_first_cached(&em_tree->root);
+	while (!RB_EMPTY_ROOT(&em_tree->root)) {
+		node = rb_first(&em_tree->root);
 		em = rb_entry(node, struct extent_map, rb_node);
 		remove_extent_mapping(inode, em);
 
@@ -551,7 +551,7 @@ static int validate_range(struct extent_map_tree *em_tree, int index)
 	struct rb_node *n;
 	int i;
 
-	for (i = 0, n = rb_first_cached(&em_tree->root);
+	for (i = 0, n = rb_first(&em_tree->root);
 	     valid_ranges[index][i].len && n;
 	     i++, n = rb_next(n)) {
 		struct extent_map *entry = rb_entry(n, struct extent_map, rb_node);