[PATCH] android, lmk: Send SIGKILL before setting TIF_MEMDIE.

[PATCH] android, lmk: Send SIGKILL before setting TIF_MEMDIE.

[Tetsuo Handa]

It was observed that setting TIF_MEMDIE before sending SIGKILL at
oom_kill_process() allows memory reserves to be depleted by allocations
which are not needed for terminating the OOM victim.

This patch reverts commit 6bc2b856bb7c ("staging: android: lowmemorykiller:
set TIF_MEMDIE before send kill sig"), for oom_kill_process() was updated
to send SIGKILL before setting TIF_MEMDIE.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 drivers/staging/android/lowmemorykiller.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/drivers/staging/android/lowmemorykiller.c b/drivers/staging/android/lowmemorykiller.c
index 872bd60..569d12c 100644
--- a/drivers/staging/android/lowmemorykiller.c
+++ b/drivers/staging/android/lowmemorykiller.c
@@ -157,26 +157,22 @@ static unsigned long lowmem_scan(struct shrinker *s, struct shrink_control *sc)
 	}
 	if (selected) {
 		task_lock(selected);
-		if (!selected->mm) {
-			/* Already exited, cannot do mark_tsk_oom_victim() */
-			task_unlock(selected);
-			goto out;
-		}
+		send_sig(SIGKILL, selected, 0);
 		/*
 		 * FIXME: lowmemorykiller shouldn't abuse global OOM killer
 		 * infrastructure. There is no real reason why the selected
 		 * task should have access to the memory reserves.
 		 */
-		mark_oom_victim(selected);
+		if (selected->mm)
+			mark_oom_victim(selected);
 		task_unlock(selected);
 		lowmem_print(1, "send sigkill to %d (%s), adj %hd, size %d\n",
 			     selected->pid, selected->comm,
 			     selected_oom_score_adj, selected_tasksize);
 		lowmem_deathpending_timeout = jiffies + HZ;
-		send_sig(SIGKILL, selected, 0);
 		rem += selected_tasksize;
 	}
-out:
+
 	lowmem_print(4, "lowmem_scan %lu, %x, return %lu\n",
 		     sc->nr_to_scan, sc->gfp_mask, rem);
 	rcu_read_unlock();
-- 
1.8.3.1

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Re: [PATCH] android, lmk: Send SIGKILL before setting TIF_MEMDIE.

[David Rientjes]

On Sun, 6 Sep 2015, Tetsuo Handa wrote:

> It was observed that setting TIF_MEMDIE before sending SIGKILL at
> oom_kill_process() allows memory reserves to be depleted by allocations
> which are not needed for terminating the OOM victim.
> 

I don't understand what you are trying to fix.  Sending a SIGKILL first 
does not guarantee that it is handling that signal before accessing memory 
reserves.  I have no objection to the patch, but I don't think it actually 
matters either way and is relatively pointless.

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Re: [PATCH] android, lmk: Send SIGKILL before setting TIF_MEMDIE.

[Michal Hocko]

On Wed 09-09-15 14:55:59, David Rientjes wrote:
> On Sun, 6 Sep 2015, Tetsuo Handa wrote:
> 
> > It was observed that setting TIF_MEMDIE before sending SIGKILL at
> > oom_kill_process() allows memory reserves to be depleted by allocations
> > which are not needed for terminating the OOM victim.
> > 
> 
> I don't understand what you are trying to fix.  Sending a SIGKILL first 
> does not guarantee that it is handling that signal before accessing memory 
> reserves. 

Yes it doesn't guarantee that but it kicks the task from userspace so
that it cannot deplete the memory reserves from the _userspace_ under
its controll. It still can consume some amount of memory from the kernel
but that shouldn't be under control of the task.
-- 
Michal Hocko
SUSE Labs

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