writeback: do foreign inode detection iff cgroup writeback is enabled

Currently, even when a filesystem doesn't set the FS_CGROUP_WRITEBACK
flag, if the filesystem uses wbc_init_bio() and wbc_account_io(), the
foreign inode detection and migration logic still ends up activating
cgroup writeback which is unexpected.  This patch ensures that the
foreign inode detection logic stays disabled when inode_cgwb_enabled()
is false by not associating writeback_control's with bdi_writeback's.

This also avoids unnecessary operations in wbc_init_bio(),
wbc_account_io() and wbc_detach_inode() for filesystems which don't
support cgroup writeback.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

fs/fs-writeback.c

@@ -513,6 +513,11 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 void wbc_attach_and_unlock_inode(struct writeback_control *wbc,
 				 struct inode *inode)
 {
+	if (!inode_cgwb_enabled(inode)) {
+		spin_unlock(&inode->i_lock);
+		return;
+	}
+
 	wbc->wb = inode_to_wb(inode);
 	wbc->inode = inode;
 
@@ -575,11 +580,16 @@ void wbc_detach_inode(struct writeback_control *wbc)
 {
 	struct bdi_writeback *wb = wbc->wb;
 	struct inode *inode = wbc->inode;
-	u16 history = inode->i_wb_frn_history;
-	unsigned long avg_time = inode->i_wb_frn_avg_time;
-	unsigned long max_bytes, max_time;
+	unsigned long avg_time, max_bytes, max_time;
+	u16 history;
 	int max_id;
 
+	if (!wb)
+		return;
+
+	history = inode->i_wb_frn_history;
+	avg_time = inode->i_wb_frn_avg_time;
+
 	/* pick the winner of this round */
 	if (wbc->wb_bytes >= wbc->wb_lcand_bytes &&
 	    wbc->wb_bytes >= wbc->wb_tcand_bytes) {