Btrfs: bulk delete checksum items in the same leaf

Very often we have the checksums for an extent spread in multiple items
in the checksums tree, and currently the algorithm to delete them starts
by looking for them one by one and then deleting them one by one, which
is not optimal since each deletion involves shifting all the other items
in the leaf and when the leaf reaches some low threshold, to move items
off the leaf into its left and right neighbor leafs. Also, after each
item deletion we release our search path and start a new search for other
checksums items.

So optimize this by deleting in bulk all the items in the same leaf that
contain checksums for the extent being freed.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>

fs/btrfs/file-item.c

@@ -643,7 +643,33 @@ int btrfs_del_csums(struct btrfs_trans_handle *trans,
 
 		/* delete the entire item, it is inside our range */
 		if (key.offset >= bytenr && csum_end <= end_byte) {
-			ret = btrfs_del_item(trans, root, path);
+			int del_nr = 1;
+
+			/*
+			 * Check how many csum items preceding this one in this
+			 * leaf correspond to our range and then delete them all
+			 * at once.
+			 */
+			if (key.offset > bytenr && path->slots[0] > 0) {
+				int slot = path->slots[0] - 1;
+
+				while (slot >= 0) {
+					struct btrfs_key pk;
+
+					btrfs_item_key_to_cpu(leaf, &pk, slot);
+					if (pk.offset < bytenr ||
+					    pk.type != BTRFS_EXTENT_CSUM_KEY ||
+					    pk.objectid !=
+					    BTRFS_EXTENT_CSUM_OBJECTID)
+						break;
+					path->slots[0] = slot;
+					del_nr++;
+					key.offset = pk.offset;
+					slot--;
+				}
+			}
+			ret = btrfs_del_items(trans, root, path,
+					      path->slots[0], del_nr);
 			if (ret)
 				goto out;
 			if (key.offset == bytenr)