[patch 11/11 -mm v4] oom: avoid race for oom killed tasks detaching mm prior to exit

[David Rientjes <rientjes@google.com>]

Tasks detach its ->mm prior to exiting so it's possible that in progress
oom kills or already exiting tasks may be missed during the oom killer's
tasklist scan.  When an eligible task is found with either TIF_MEMDIE or
PF_EXITING set, the oom killer is supposed to be a no-op to avoid
needlessly killing additional tasks.  This closes the race between a task
detaching its ->mm and being removed from the tasklist.

Out of memory conditions as the result of memory controllers will
automatically filter tasks that have detached their ->mm (since
task_in_mem_cgroup() will return 0).  This is acceptable, however, since
memcg constrained ooms aren't the result of a lack of memory resources
but rather a limit imposed by userspace that requires a task be killed
regardless.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 mm/oom_kill.c |   12 ++++++------
 1 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -290,12 +290,6 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
 	for_each_process(p) {
 		unsigned int points;
 
-		/*
-		 * skip kernel threads and tasks which have already released
-		 * their mm.
-		 */
-		if (!p->mm)
-			continue;
 		/* skip the init task */
 		if (is_global_init(p))
 			continue;
@@ -336,6 +330,12 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
 			*ppoints = 1000;
 		}
 
+		/*
+		 * skip kernel threads and tasks which have already released
+		 * their mm.
+		 */
+		if (!p->mm)
+			continue;
 		if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
 			continue;
 

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