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Sender: Corey Minyard <tcminyard@gmail.com>
From:   minyard@acm.org
To:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org
Cc:     linux-kernel@vger.kernel.org, Corey Minyard <cminyard@mvista.com>
Subject: [PATCH] oom_kill: oom_score_adj broken for processes with small memory usage
Date:   Thu,  1 Jul 2021 07:54:30 -0500
Message-Id: <20210701125430.836308-1-minyard@acm.org>
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From: Corey Minyard <cminyard@mvista.com>

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.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
---
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.

 mm/oom_kill.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index eefd3f5fde46..4ae0b6193bcd 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -233,8 +233,11 @@ 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;
-- 
2.25.1