memcg: fix mem_cgroup_print_bad_page

If DEBUG_VM, mem_cgroup_print_bad_page() is called whenever bad_page()
shows a "Bad page state" message, removes page from circulation, adds a
taint and continues.  This is at a very low level, often when a spinlock
is held (sometimes when page table lock is held, for example).

We want to recover from this badness, not make it worse: we must not
kmalloc memory here, we must not do a cgroup path lookup via dubious
pointers.  No doubt that code was useful to debug a particular case at one
time, and may be again, but take it out of the mainline kernel.
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3364,23 +3364,8 @@ void mem_cgroup_print_bad_page(struct page *page)
 
 	pc = lookup_page_cgroup_used(page);
 	if (pc) {
-		int ret = -1;
-		char *path;
-
-		printk(KERN_ALERT "pc:%p pc->flags:%lx pc->mem_cgroup:%p",
+		printk(KERN_ALERT "pc:%p pc->flags:%lx pc->mem_cgroup:%p\n",
 		       pc, pc->flags, pc->mem_cgroup);
-
-		path = kmalloc(PATH_MAX, GFP_KERNEL);
-		if (path) {
-			rcu_read_lock();
-			ret = cgroup_path(pc->mem_cgroup->css.cgroup,
-							path, PATH_MAX);
-			rcu_read_unlock();
-		}
-
-		printk(KERN_CONT "(%s)\n",
-				(ret < 0) ? "cannot get the path" : path);
-		kfree(path);
 	}
 }
 #endif