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>
4e660ca3a9