Btrfs: use the flusher threads for delalloc throttling

We have a fairly complex set of loops around walking our list of
delalloc inodes when we find metadata delalloc space running low.
It doesn't work very well, can use large amounts of CPU and doesn't
do very efficient writeback.

This switches us to kick the bdi flusher threads instead.  All dirty
data in btrfs is accounted as delalloc data, so this is very similar
in terms of what it writes, but we're able to just kick off the IO
and wait for progress.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -3328,15 +3328,14 @@ static int shrink_delalloc(struct btrfs_trans_handle *trans,
 	u64 reserved;
 	u64 max_reclaim;
 	u64 reclaimed = 0;
-	int no_reclaim = 0;
 	int pause = 1;
-	int ret;
+	int nr_pages = (2 * 1024 * 1024) >> PAGE_CACHE_SHIFT;
 
 	block_rsv = &root->fs_info->delalloc_block_rsv;
 	space_info = block_rsv->space_info;
-	spin_lock(&space_info->lock);
+
+	smp_mb();
 	reserved = space_info->bytes_reserved;
-	spin_unlock(&space_info->lock);
 
 	if (reserved == 0)
 		return 0;
@@ -3344,20 +3343,11 @@ static int shrink_delalloc(struct btrfs_trans_handle *trans,
 	max_reclaim = min(reserved, to_reclaim);
 
 	while (1) {
-		ret = btrfs_start_one_delalloc_inode(root, trans ? 1 : 0, sync);
-		if (!ret) {
-			if (no_reclaim > 2)
-				break;
-			no_reclaim++;
-			__set_current_state(TASK_INTERRUPTIBLE);
-			schedule_timeout(pause);
-			pause <<= 1;
-			if (pause > HZ / 10)
-				pause = HZ / 10;
-		} else {
-			no_reclaim = 0;
-			pause = 1;
-		}
+		/* have the flusher threads jump in and do some IO */
+		smp_mb();
+		nr_pages = min_t(unsigned long, nr_pages,
+		       root->fs_info->delalloc_bytes >> PAGE_CACHE_SHIFT);
+		writeback_inodes_sb_nr_if_idle(root->fs_info->sb, nr_pages);
 
 		spin_lock(&space_info->lock);
 		if (reserved > space_info->bytes_reserved)
@@ -3370,6 +3360,13 @@ static int shrink_delalloc(struct btrfs_trans_handle *trans,
 
 		if (trans && trans->transaction->blocked)
 			return -EAGAIN;
+
+		__set_current_state(TASK_INTERRUPTIBLE);
+		schedule_timeout(pause);
+		pause <<= 1;
+		if (pause > HZ / 10)
+			pause = HZ / 10;
+
 	}
 	return reclaimed >= to_reclaim;
 }