Btrfs: don't wait for all the async delalloc when shrinking delalloc

It was very likely that there were lots of async delalloc pages in the
filesystem, if we waited until all the pages were flushed, we would be
blocked for a long time, and the performance would also drop down.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/extent-tree.c

@@ -4079,9 +4079,19 @@ static void shrink_delalloc(struct btrfs_root *root, u64 to_reclaim, u64 orig,
 		 * We need to wait for the async pages to actually start before
 		 * we do anything.
 		 */
-		wait_event(root->fs_info->async_submit_wait,
-			   !atomic_read(&root->fs_info->async_delalloc_pages));
+		max_reclaim = atomic_read(&root->fs_info->async_delalloc_pages);
+		if (!max_reclaim)
+			goto skip_async;
+
+		if (max_reclaim <= nr_pages)
+			max_reclaim = 0;
+		else
+			max_reclaim -= nr_pages;
 
+		wait_event(root->fs_info->async_submit_wait,
+			   atomic_read(&root->fs_info->async_delalloc_pages) <=
+			   (int)max_reclaim);
+skip_async:
 		if (!trans)
 			flush = BTRFS_RESERVE_FLUSH_ALL;
 		else