[Devel] [PATCH rh7 1/6] mm, oom: remove unnecessary exit_state check

[Vladimir Davydov]

From: David Rientjes <rientjes at google.com>

The oom killer scans each process and determines whether it is eligible
for oom kill or whether the oom killer should abort because of
concurrent memory freeing.  It will abort when an eligible process is
found to have TIF_MEMDIE set, meaning it has already been oom killed and
we're waiting for it to exit.

Processes with task->mm == NULL should not be considered because they
are either kthreads or have already detached their memory and killing
them would not lead to memory freeing.  That memory is only freed after
exit_mm() has returned, however, and not when task->mm is first set to
NULL.

Clear TIF_MEMDIE after exit_mm()'s mmput() so that an oom killed process
is no longer considered for oom kill, but only until exit_mm() has
returned.  This was fragile in the past because it relied on
exit_notify() to be reached before no longer considering TIF_MEMDIE
processes.

Signed-off-by: David Rientjes <rientjes at google.com>
Cc: Oleg Nesterov <oleg at redhat.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit fb794bcbb4e5552242f9a4c5e1ffe4c6da29a968)
Signed-off-by: Vladimir Davydov <vdavydov at parallels.com>
---
 kernel/exit.c | 1 +
 mm/oom_kill.c | 2 --
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index dbc8f77952e7..90feb5fbdb10 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -520,6 +520,7 @@ static void exit_mm(struct task_struct * tsk)
 	task_unlock(tsk);
 	mm_update_next_owner(mm);
 	mmput(mm);
+	clear_thread_flag(TIF_MEMDIE);
 }
 
 /*
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 5413a4413a3c..57d9f3eff4a4 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -258,8 +258,6 @@ enum oom_scan_t oom_scan_process_thread(struct task_struct *task,
 		unsigned long totalpages, const nodemask_t *nodemask,
 		bool force_kill, bool ignore_memcg_guarantee)
 {
-	if (task->exit_state)
-		return OOM_SCAN_CONTINUE;
 	if (oom_unkillable_task(task, NULL, nodemask))
 		return OOM_SCAN_CONTINUE;
 
-- 
2.1.4