cpuset, mm: fix TIF_MEMDIE check in cpuset_change_task_nodemask

Commit c0ff745 ("cpuset,mm: fix no node to alloc memory when
changing cpuset's mems") has added TIF_MEMDIE and PF_EXITING check but
it is checking the flag on the current task rather than the given one.

This doesn't make much sense and it is actually wrong.  If the current
task which updates the nodemask of a cpuset got killed by the OOM killer
then a part of the cpuset cgroup processes would have incompatible
nodemask which is surprising to say the least.

The comment suggests the intention was to skip oom victim or an exiting
task so we should be checking the given task.  But even then it would be
layering violation because it is the memory allocator to interpret the
TIF_MEMDIE meaning.  Simply drop both checks.  All tasks in the cpuset
should simply follow the same mask.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Miao Xie <[email protected]>
Cc: Miao Xie <[email protected]>
Cc: "Rafael J. Wysocki" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

kernel/cpuset.c

@@ -1034,15 +1034,6 @@ static void cpuset_change_task_nodemask(struct task_struct *tsk,
 {
 	bool need_loop;
 
-	/*
-	 * Allow tasks that have access to memory reserves because they have
-	 * been OOM killed to get memory anywhere.
-	 */
-	if (unlikely(test_thread_flag(TIF_MEMDIE)))
-		return;
-	if (current->flags & PF_EXITING) /* Let dying task have memory */
-		return;
-
 	task_lock(tsk);
 	/*
 	 * Determine if a loop is necessary if another thread is doing