oom_kill: oom_score_adj broken for processes with small memory usage

If you have a process with less than 1000 totalpages, the calculation:

  adj = (long)p->signal->oom_score_adj;
  ...
  adj *= totalpages / 1000;

will always result in adj being zero no matter what oom_score_adj is,
which could result in the wrong process being picked for killing.

Fix by adding 1000 to totalpages before dividing.

I ran across this trying to diagnose another problem where I set up a
cgroup with a small amount of memory and couldn't get a test program to
work right.

I'm not sure this is quite right, to keep closer to the current behavior
you could do:

	if (totalpages >= 1000) adj *= totalpages / 1000;

but that would map 0-1999 to the same value.  But this at least shows the
issue.  I can provide a test program the shows the issue, but I think it's
pretty obvious from the code.

Link: https://lkml.kernel.org/r/20210701125430.836308-1-minyard@acm.org
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 831340e7ad8b4721a066c7d7d850e9b841ab38ff..431d38c3bba8ef35861e21f43e471c22bc753576 100644 (file)
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -234,8 +234,11 @@ long oom_badness(struct task_struct *p, unsigned long totalpages)
                mm_pgtables_bytes(p->mm) / PAGE_SIZE;
        task_unlock(p);
 
-       /* Normalize to oom_score_adj units */
-       adj *= totalpages / 1000;
+       /*
+        * Normalize to oom_score_adj units.  You should never
+        * multiply by zero here, or oom_score_adj will not work.
+        */
+       adj *= (totalpages + 1000) / 1000;
        points += adj;
 
        return points;