[Devel] [PATCH rh7 2/2] exit: clear TIF_MEMDIE after exit_task_work

[Vladimir Davydov]

An mm_struct may be pinned by a file. An example is vhost-net device
created by a qemu/kvm (see vhost_net_ioctl -> vhost_net_set_owner ->
vhost_dev_set_owner). If such process gets OOM-killed, the reference to
its mm_struct will only be released from exit_task_work -> ____fput ->
__fput -> vhost_net_release -> vhost_dev_cleanup, which is called after
exit_mmap, where TIF_MEMDIE is cleared. As a result, we can start
selecting the next victim before giving the last one a chance to free
its memory. In practice, this leads to killing several VMs along with
the fattest one.

https://jira.sw.ru/browse/PSBM-44683

Signed-off-by: Vladimir Davydov <vdavydov at virtuozzo.com>
---
 kernel/exit.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 1cc765b48bd7..2715dc1dc0f5 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -520,8 +520,6 @@ static void exit_mm(struct task_struct * tsk)
 	task_unlock(tsk);
 	mm_update_next_owner(mm);
 	mmput(mm);
-	if (test_thread_flag(TIF_MEMDIE))
-		exit_oom_victim();
 }
 
 /*
@@ -819,6 +817,10 @@ void do_exit(long code)
 	exit_fs(tsk);
 	exit_task_namespaces(tsk);
 	exit_task_work(tsk);
+
+	if (test_thread_flag(TIF_MEMDIE))
+		exit_oom_victim();
+
 	check_stack_usage();
 	exit_thread();
 
-- 
2.1.4