Re: [PATCH 1/3] mm,oom: Reverse the order of setting TIF_MEMDIE and sending SIGKILL.

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 20 Sep 2015 11:04:43 +0900 Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:

> 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.
> 
> ...
>
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -554,6 +554,8 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  
>  	/* mm cannot safely be dereferenced after task_unlock(victim) */
>  	mm = victim->mm;
> +	/* Send SIGKILL before setting TIF_MEMDIE. */
> +	do_send_sig_info(SIGKILL, SEND_SIG_FORCED, victim, true);

The patch looks good, but the comment is poor.  It says what the code
does (which is obvious anyway) but fails to describe *why* the code is
this way, which is what the reader wants to understand.

In fact the comment seems rather misleading, because we could retain
the current ordering:

	mark_oom_victim(...);
	do_send_sig_info(...);

and still achieve this patch's objectives?

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