btrfs: remove unnecessary EXTENT_UPTODATE state in buffered I/O path

After we copied data to page cache in buffered I/O, we
1. Insert a EXTENT_UPTODATE state into inode's io_tree, by
   endio_readpage_release_extent(), set_extent_delalloc() or
   set_extent_defrag().
2. Set page uptodate before we unlock the page.

But the only place we check io_tree's EXTENT_UPTODATE state is in
btrfs_do_readpage(). We know we enter btrfs_do_readpage() only when we
have a non-uptodate page, so it is unnecessary to set EXTENT_UPTODATE.

For example, when performing a buffered random read:

	fio --rw=randread --ioengine=libaio --direct=0 --numjobs=4 \
		--filesize=32G --size=4G --bs=4k --name=job \
		--filename=/mnt/file --name=job

Then check how many extent_state in io_tree:

	cat /proc/slabinfo | grep btrfs_extent_state | awk '{print $2}'

w/o this patch, we got 640567 btrfs_extent_state.
w/  this patch, we got    204 btrfs_extent_state.

Maintaining such a big tree brings overhead since every I/O needs to insert
EXTENT_LOCKED, insert EXTENT_UPTODATE, then remove EXTENT_LOCKED. And in
every insert or remove, we need to lock io_tree, do tree search, alloc or
dealloc extent states. By removing unnecessary EXTENT_UPTODATE, we keep
io_tree in a minimal size and reduce overhead when performing buffered I/O.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: Ethan Lien <ethanlien@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 0 insertions, 3 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index b04ce7a299ad..0a7770571729 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6868,7 +6868,6 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,
 	struct btrfs_key found_key;
 	struct extent_map *em = NULL;
 	struct extent_map_tree *em_tree = &inode->extent_tree;
-	struct extent_io_tree *io_tree = &inode->io_tree;
 
 	read_lock(&em_tree->lock);
 	em = lookup_extent_mapping(em_tree, start, len);
@@ -7031,8 +7030,6 @@ next:
 			}
 			flush_dcache_page(page);
 		}
-		set_extent_uptodate(io_tree, em->start,
-				    extent_map_end(em) - 1, NULL, GFP_NOFS);
 		goto insert;
 	}
 not_found: