Btrfs: cache values for locking extents

Many of the btrfs extent state tree users follow the same pattern.
They lock an extent range in the tree, do some operation and then
unlock.

This translates to at least 2 rbtree searches, and maybe more if they
are doing operations on the extent state tree.  A locked extent
in the tree isn't going to be merged or changed, and so we can
safely return the extent state structure as a cached handle.

This changes set_extent_bit to give back a cached handle, and also
changes both set_extent_bit and clear_extent_bit to use the cached
handle if it is available.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.h

@@ -143,7 +143,7 @@ int try_release_extent_state(struct extent_map_tree *map,
 			     gfp_t mask);
 int lock_extent(struct extent_io_tree *tree, u64 start, u64 end, gfp_t mask);
 int lock_extent_bits(struct extent_io_tree *tree, u64 start, u64 end,
-		     int bits, gfp_t mask);
+		     int bits, struct extent_state **cached, gfp_t mask);
 int unlock_extent(struct extent_io_tree *tree, u64 start, u64 end, gfp_t mask);
 int try_lock_extent(struct extent_io_tree *tree, u64 start, u64 end,
 		    gfp_t mask);
@@ -161,7 +161,8 @@ int test_range_bit(struct extent_io_tree *tree, u64 start, u64 end,
 int clear_extent_bits(struct extent_io_tree *tree, u64 start, u64 end,
 		      int bits, gfp_t mask);
 int clear_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
-		     int bits, int wake, int delete, gfp_t mask);
+		     int bits, int wake, int delete, struct extent_state **cached,
+		     gfp_t mask);
 int set_extent_bits(struct extent_io_tree *tree, u64 start, u64 end,
 		    int bits, gfp_t mask);
 int set_extent_uptodate(struct extent_io_tree *tree, u64 start, u64 end,

fs/btrfs/inode.c

@@ -854,7 +854,7 @@ static int cow_file_range_async(struct inode *inode, struct page *locked_page,
 	int limit = 10 * 1024 * 1042;
 
 	clear_extent_bit(&BTRFS_I(inode)->io_tree, start, end, EXTENT_LOCKED |
-			 EXTENT_DELALLOC, 1, 0, GFP_NOFS);
+			 EXTENT_DELALLOC, 1, 0, NULL, GFP_NOFS);
 	while (start < end) {
 		async_cow = kmalloc(sizeof(*async_cow), GFP_NOFS);
 		async_cow->inode = inode;
@@ -4420,7 +4420,7 @@ static void btrfs_invalidatepage(struct page *page, unsigned long offset)
 		 */
 		clear_extent_bit(tree, page_start, page_end,
 				 EXTENT_DIRTY | EXTENT_DELALLOC |
-				 EXTENT_LOCKED, 1, 0, GFP_NOFS);
+				 EXTENT_LOCKED, 1, 0, NULL, GFP_NOFS);
 		btrfs_finish_ordered_io(page->mapping->host,
 					page_start, page_end);
 		btrfs_put_ordered_extent(ordered);
@@ -4429,7 +4429,7 @@ static void btrfs_invalidatepage(struct page *page, unsigned long offset)
 	clear_extent_bit(tree, page_start, page_end,
 		 EXTENT_LOCKED | EXTENT_DIRTY | EXTENT_DELALLOC |
 		 EXTENT_ORDERED,
-		 1, 1, GFP_NOFS);
+		 1, 1, NULL, GFP_NOFS);
 	__btrfs_releasepage(page, GFP_NOFS);
 
 	ClearPageChecked(page);