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Subject[PATCH] oom_kill: oom_score_adj broken for processes with small memory usage
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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
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