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Subject[patch 7/7] oom: prevent possible OOM_DISABLE livelock
It is currently possible to livelock the oom killer if a task is chosen
for oom kill and another thread sharing the same memory has an oom_adj
value of OOM_DISABLE. This occurs because oom_kill_task() repeatedly
returns 1 and refuses to kill the chosen task while select_bad_process()
will repeatedly chooses the same task during the next retry.

This moves the check for OOM_DISABLE to the badness heuristic while
holding task_lock(). Badness scores of 0 are now explicitly prohibited
from being oom killed and since the oom_adj value is a characteristic of
an mm and not a task, it is no longer necessary to check the oom_adj
value for threads sharing the same memory (except when simply issuing
SIGKILLs for threads in other thread groups).

Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: David Rientjes <rientjes@google.com>
---
mm/oom_kill.c | 40 ++++++++++------------------------------
1 files changed, 10 insertions(+), 30 deletions(-)
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -67,6 +67,10 @@ unsigned long badness(struct task_struct *p, unsigned long uptime)
return 0;
}
oom_adj = mm->oom_adj;
+ if (oom_adj == OOM_DISABLE) {
+ task_unlock(p);
+ return 0;
+ }

/*
* The memory size of the process is the basis for the badness.
@@ -253,13 +257,8 @@ static struct task_struct *select_bad_process(unsigned long *ppoints,
*ppoints = ULONG_MAX;
}

- task_lock(p);
- if (p->mm && p->mm->oom_adj == OOM_DISABLE)
- continue;
- task_unlock(p);
-
points = badness(p, uptime.tv_sec);
- if (points > *ppoints || !chosen) {
+ if (points > *ppoints) {
chosen = p;
*ppoints = points;
}
@@ -352,32 +351,13 @@ static int oom_kill_task(struct task_struct *p)
struct mm_struct *mm;
struct task_struct *g, *q;

+ task_lock(p);
mm = p->mm;
-
- /* WARNING: mm may not be dereferenced since we did not obtain its
- * value from get_task_mm(p). This is OK since all we need to do is
- * compare mm to q->mm below.
- *
- * Furthermore, even if mm contains a non-NULL value, p->mm may
- * change to NULL at any time since we do not hold task_lock(p).
- * However, this is of no concern to us.
- */
-
- if (mm == NULL)
+ if (!mm || mm->oom_adj == OOM_DISABLE) {
+ task_unlock(p);
return 1;
-
- /*
- * Don't kill the process if any threads are set to OOM_DISABLE
- */
- do_each_thread(g, q) {
- task_lock(q);
- if (q->mm == mm && q->mm && q->mm->oom_adj == OOM_DISABLE) {
- task_unlock(q);
- return 1;
- }
- task_unlock(q);
- } while_each_thread(g, q);
-
+ }
+ task_unlock(p);
__oom_kill_task(p, 1);

/*

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