fs/writeback: Attach inode's wb to root if needed

There might have tons of files queued in the writeback, awaiting for
writing back. Unfortunately, the writeback's cgroup has been dead. In
this case, we reassociate the inode with another writeback, but we
possibly can't because the writeback associated with the dead cgroup is
the only valid one. In this case, the new writeback is allocated,
initialized and associated with the inode in the non-stopping fashion
until all data resident in the inode's page cache are flushed to disk.
It causes unnecessary high system load.

This fixes the issue by enforce moving the inode to root cgroup when the
previous binding cgroup becomes dead. With it, no more unnecessary
writebacks are created, populated and the system load decreased by about
6x in the test case we carried out:
    Without the patch: 30% system load
    With the patch:    5%  system load
Signed-off-by: Nluanshi <zhangliguang@linux.alibaba.com>
Signed-off-by: NJiufei Xue <jiufei.xue@linux.alibaba.com>
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>

fs/fs-writeback.c

@@ -704,6 +704,16 @@ void wbc_detach_inode(struct writeback_control *wbc)
 	inode->i_wb_frn_avg_time = min(avg_time, (unsigned long)U16_MAX);
 	inode->i_wb_frn_history = history;
 
+	/*
+	 * Without wb list lock i_wb can switch at any point, so it can
+	 * judge on the wrong wb anyway.
+	 *
+	 * The wb is switched to the root memcg unconditionally. We expect
+	 * the correct wb (best candidate) is picked up in next round.
+	 */
+	if (wb == inode->i_wb && wb_dying(wb) && !(inode->i_state & I_DIRTY_ALL))
+		inode_switch_wbs(inode, root_mem_cgroup->css.id);
+
 	wb_put(wbc->wb);
 	wbc->wb = NULL;
 }