Re: [patch 03/12] mm: oom_kill: switch test-and-clear of known TIF_MEMDIE to clear

David Rientjes
2015-03-26T03:31:55Z

On Wed, 25 Mar 2015, Johannes Weiner wrote:

> exit_oom_victim() already knows that TIF_MEMDIE is set, and nobody
> else can clear it concurrently. Use clear_thread_flag() directly.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

For the oom killer, that's true because of task_lock(): we always only set
TIF_MEMDIE when there is a valid p->mm and it's cleared in the exit path
after the unlock, acting as a barrier, when p->mm is set to NULL so it's
no longer a valid victim. So that part is fine.

The problem is the android low memory killer that does
mark_tsk_oom_victim() without the protection of task_lock(), it's just rcu
protected so the reference to the task itself is guaranteed to still be
valid.

I assume that's why Michal implemented it this way and added the comment
to the lmk in commit 49550b605587 ("oom: add helpers for setting and
clearing TIF_MEMDIE") to avoid TIF_MEMDIE entirely there.

> ---
> mm/oom_kill.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index b2f081fe4b1a..4b9547be9170 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -435,8 +435,7 @@ void mark_oom_victim(struct task_struct *tsk)
> */
> void exit_oom_victim(void)
> {
> - if (!test_and_clear_thread_flag(TIF_MEMDIE))
> - return;
> + clear_thread_flag(TIF_MEMDIE);
>
> down_read(&oom_sem);
> /*