Re: [REPOST] [PATCH 2/2] mm,oom: Reverse the order of setting TIF_MEMDIE and sending SIGKILL.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Michal Hocko wrote:
> The comment could have been a bit better because it doesn't exmplain
> why but this is much much better.
>
> I guess you want to add Andrew to the CC because he might miss it this
> way easily.
> 
> I would also consider marking this for stable.
> 
OK. Here is an updated patch. Can you carry this patch through your git
tree (because this patch depends on previous "[PATCH 1/2] mm,oom: Fix
potentially killing unrelated process.")?
----------
>From e9077d84d9d1e32b91b8e44534dd6fab494bbbe2 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Thu, 27 Aug 2015 19:26:35 +0900
Subject: [PATCH 2/2] mm,oom: Reverse the order of setting TIF_MEMDIE and sending SIGKILL.

It was confirmed that a local unprivileged user can consume all memory
reserves and hang up that system using time lag between the OOM killer
sets TIF_MEMDIE on an OOM victim and sends SIGKILL to that victim, for
printk() inside for_each_process() loop at oom_kill_process() can consume
many seconds when there are many thread groups sharing the same memory.


Before starting oom-depleter process:

    Node 0 DMA: 3*4kB (UM) 6*8kB (U) 4*16kB (UEM) 0*32kB 0*64kB 1*128kB (M) 2*256kB (EM) 2*512kB (UE) 2*1024kB (EM) 1*2048kB (E) 1*4096kB (M) = 9980kB
    Node 0 DMA32: 31*4kB (UEM) 27*8kB (UE) 32*16kB (UE) 13*32kB (UE) 14*64kB (UM) 7*128kB (UM) 8*256kB (UM) 8*512kB (UM) 3*1024kB (U) 4*2048kB (UM) 362*4096kB (UM) = 1503220kB

As of invoking the OOM killer:

    Node 0 DMA: 11*4kB (UE) 8*8kB (UEM) 6*16kB (UE) 2*32kB (EM) 0*64kB 1*128kB (U) 3*256kB (UEM) 2*512kB (UE) 3*1024kB (UEM) 1*2048kB (U) 0*4096kB = 7308kB
    Node 0 DMA32: 1049*4kB (UEM) 507*8kB (UE) 151*16kB (UE) 53*32kB (UEM) 83*64kB (UEM) 52*128kB (EM) 25*256kB (UEM) 11*512kB (M) 6*1024kB (UM) 1*2048kB (M) 0*4096kB = 44556kB

Between the thread group leader got TIF_MEMDIE and receives SIGKILL:

    Node 0 DMA: 0*4kB 0*8kB 0*16kB 0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 0kB
    Node 0 DMA32: 0*4kB 0*8kB 0*16kB 0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 0kB


The oom-depleter's thread group leader which got TIF_MEMDIE started
memset() in user space after the OOM killer set TIF_MEMDIE, and it
was free to abuse ALLOC_NO_WATERMARKS by TIF_MEMDIE for memset()
in user space until SIGKILL is delivered. If SIGKILL is delivered
before TIF_MEMDIE is set, the oom-depleter can terminate without
touching memory reserves.

Although the possibility of hitting this time lag is very small for 3.19
and earlier kernels because TIF_MEMDIE is set immediately before sending
SIGKILL, preemption or long interrupts (an extreme example is SysRq-t)
can step between and allow memory allocations which are not needed for
terminating the OOM victim.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: stable <stable@vger.kernel.org> [4.0+]
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/oom_kill.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 5249e7e..408aa8e 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -555,6 +555,8 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 	/* Get a reference to safely compare mm after task_unlock(victim) */
 	mm = victim->mm;
 	atomic_inc(&mm->mm_users);
+	/* Send SIGKILL before setting TIF_MEMDIE. */
+	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
 	mark_oom_victim(victim);
 	pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
 		task_pid_nr(victim), victim->comm, K(victim->mm->total_vm),
@@ -586,7 +588,6 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 		}
 	rcu_read_unlock();
 
-	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);
 	mmput(mm);
 	put_task_struct(victim);
 }
-- 
1.8.3.1

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