Btrfs: Handle writeback under high memory pressure better

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 39be6ba..3c00f96 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -961,7 +961,7 @@
 
 	level = btrfs_header_level(root->node);
 
-	if (num_bytes >= 96 * 1024 * 1024 && hint_byte) {
+	if (num_bytes >= 32 * 1024 * 1024 && hint_byte) {
 		data = BTRFS_BLOCK_GROUP_MIXED;
 	}