Btrfs: Improve metadata read latencies

This fixes latency problems on metadata reads by making sure they
don't go through the async submit queue, and by tuning down the amount
of readahead done during btree searches.

Also, the btrfs bdi congestion function is tuned to ignore the
number of pending async bios and checksums pending.  There is additional
code that throttles new async bios now and the congestion function
doesn't need to worry about it anymore.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -1285,16 +1285,16 @@ static noinline void reada_for_search(struct btrfs_root *root,
 		}
 		search = btrfs_node_blockptr(node, nr);
 		if ((search >= lowest_read && search <= highest_read) ||
-		    (search < lowest_read && lowest_read - search <= 32768) ||
-		    (search > highest_read && search - highest_read <= 32768)) {
+		    (search < lowest_read && lowest_read - search <= 16384) ||
+		    (search > highest_read && search - highest_read <= 16384)) {
 			readahead_tree_block(root, search, blocksize,
 				     btrfs_node_ptr_generation(node, nr));
 			nread += blocksize;
 		}
 		nscan++;
-		if (path->reada < 2 && (nread > (256 * 1024) || nscan > 32))
+		if (path->reada < 2 && (nread > (64 * 1024) || nscan > 32))
 			break;
-		if(nread > (1024 * 1024) || nscan > 128)
+		if(nread > (256 * 1024) || nscan > 128)
 			break;
 
 		if (search < lowest_read)

fs/btrfs/disk-io.c

@@ -605,7 +605,7 @@ static int btree_submit_bio_hook(struct inode *inode, int rw, struct bio *bio,
 		BUG_ON(ret);
 
 		return btrfs_map_bio(BTRFS_I(inode)->root, rw, bio,
-				     mirror_num, 1);
+				     mirror_num, 0);
 	}
 	return btrfs_wq_submit_bio(BTRFS_I(inode)->root->fs_info,
 				   inode, rw, bio, mirror_num, 0,
@@ -1139,11 +1139,11 @@ static int btrfs_congested_fn(void *congested_data, int bdi_bits)
 	struct list_head *cur;
 	struct btrfs_device *device;
 	struct backing_dev_info *bdi;
-
+#if 0
 	if ((bdi_bits & (1 << BDI_write_congested)) &&
 	    btrfs_congested_async(info, 0))
 		return 1;
-
+#endif
 	list_for_each(cur, &info->fs_devices->devices) {
 		device = list_entry(cur, struct btrfs_device, dev_list);
 		if (!device->bdev)