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

[ethanlien <ethanlien@synology.com>]

From: Ethan Lien <ethanlien@synology.com>

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.

Signed-off-by: Ethan Lien <ethanlien@synology.com>
Reviewed-by: Robbie Ko <robbieko@synology.com>
---
 fs/btrfs/extent-io-tree.h | 4 ++--
 fs/btrfs/extent_io.c      | 3 ---
 2 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/extent-io-tree.h b/fs/btrfs/extent-io-tree.h
index c3eb52dbe61c..53ae849d0248 100644
--- a/fs/btrfs/extent-io-tree.h
+++ b/fs/btrfs/extent-io-tree.h
@@ -211,7 +211,7 @@ static inline int set_extent_delalloc(struct extent_io_tree *tree, u64 start,
 				      struct extent_state **cached_state)
 {
 	return set_extent_bit(tree, start, end,
-			      EXTENT_DELALLOC | EXTENT_UPTODATE | extra_bits,
+			      EXTENT_DELALLOC | extra_bits,
 			      0, NULL, cached_state, GFP_NOFS, NULL);
 }
 
@@ -219,7 +219,7 @@ static inline int set_extent_defrag(struct extent_io_tree *tree, u64 start,
 		u64 end, struct extent_state **cached_state)
 {
 	return set_extent_bit(tree, start, end,
-			      EXTENT_DELALLOC | EXTENT_UPTODATE | EXTENT_DEFRAG,
+			      EXTENT_DELALLOC | EXTENT_DEFRAG,
 			      0, NULL, cached_state, GFP_NOFS, NULL);
 }
 
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index bfae67c593c5..e0f0a39cd6eb 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2924,9 +2924,6 @@ static void endio_readpage_release_extent(struct processed_extent *processed,
 	 * Now we don't have range contiguous to the processed range, release
 	 * the processed range now.
 	 */
-	if (processed->uptodate && tree->track_uptodate)
-		set_extent_uptodate(tree, processed->start, processed->end,
-				    &cached, GFP_ATOMIC);
 	unlock_extent_cached_atomic(tree, processed->start, processed->end,
 				    &cached);
 
-- 
2.17.1