oom: task->mm == NULL doesn't mean the memory was freed

exit_mm() sets ->mm == NULL then it does mmput()->exit_mmap() which
frees the memory.

However select_bad_process() checks ->mm != NULL before TIF_MEMDIE,
so it continues to kill other tasks even if we have the oom-killed
task freeing its memory.

Change select_bad_process() to check ->mm after TIF_MEMDIE, but skip
the tasks which have already passed exit_notify() to ensure a zombie
with TIF_MEMDIE set can't block oom-killer. Alternatively we could
probably clear TIF_MEMDIE after exit_mmap().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index eafff89b3dd6..626303b52f3c 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -303,7 +303,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
         do_each_thread(g, p) {
                 unsigned int points;
 
-                if (!p->mm)
+                if (p->exit_state)
                         continue;
                 if (oom_unkillable_task(p, mem, nodemask))
                         continue;
@@ -319,6 +319,8 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
                  */
                 if (test_tsk_thread_flag(p, TIF_MEMDIE))
                         return ERR_PTR(-1UL);
+                if (!p->mm)
+                        continue;
 
                 if (p->flags & PF_EXITING) {
                         /*