mm, oom: remove 3% bonus for CAP_SYS_ADMIN processes

Since the 2.6 kernel, the oom killer has slightly biased away from
CAP_SYS_ADMIN processes by discounting some of its memory usage in
comparison to other processes.

This has always been implicit and nothing exactly relies on the
behavior.

Gaurav notices that __task_cred() can dereference a potentially freed
pointer if the task under consideration is exiting because a reference
to the task_struct is not held.

Remove the CAP_SYS_ADMIN bias so that all processes are treated equally.

If any CAP_SYS_ADMIN process would like to be biased against, it is
always allowed to adjust /proc/pid/oom_score_adj.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: David Rientjes <[email protected]>
Reported-by: Gaurav Kohli <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/oom_kill.c

@@ -226,13 +226,6 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 		mm_pgtables_bytes(p->mm) / PAGE_SIZE;
 	task_unlock(p);
 
-	/*
-	 * Root processes get 3% bonus, just like the __vm_enough_memory()
-	 * implementation used by LSMs.
-	 */
-	if (has_capability_noaudit(p, CAP_SYS_ADMIN))
-		points -= (points * 3) / 100;
-
 	/* Normalize to oom_score_adj units */
 	adj *= totalpages / 1000;
 	points += adj;