fix race, add pid & comm to message

On Tue, 10 Nov 2009, akpm@linux-foundation.org wrote:

> diff -puN mm/oom_kill.c~oom-kill-show-virtual-size-and-rss-information-of-the-killed-process mm/oom_kill.c
> --- a/mm/oom_kill.c~oom-kill-show-virtual-size-and-rss-information-of-the-killed-process
> +++ a/mm/oom_kill.c
> @@ -352,6 +352,8 @@ static void dump_header(gfp_t gfp_mask,
>  		dump_tasks(mem);
>  }
>
> +#define K(x) ((x) << (PAGE_SHIFT-10))
> +
>  /*
>   * Send SIGKILL to the selected  process irrespective of  CAP_SYS_RAW_IO
>   * flag though it's unlikely that  we select a process with CAP_SYS_RAW_IO
> @@ -371,9 +373,16 @@ static void __oom_kill_task(struct task_
>  		return;
>  	}
>
> -	if (verbose)
> -		printk(KERN_ERR "Killed process %d (%s)\n",
> -				task_pid_nr(p), p->comm);
> +	if (verbose) {
> +		task_lock(p);
> +		printk(KERN_ERR "Killed process %d (%s) "
> +		       "vsz:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
> +		       task_pid_nr(p), p->comm,
> +		       K(p->mm->total_vm),
> +		       K(get_mm_counter(p->mm, anon_rss)),
> +		       K(get_mm_counter(p->mm, file_rss)));
> +		task_unlock(p);
> +	}
>
>  	/*
>  	 * We give our sacrificial lamb high priority and access to

There's a race there which can dereference a NULL p->mm.

p->mm is protected by task_lock(), but there's no check added here that
ensures p->mm is still valid.  The previous check for !p->mm in
__oom_kill_task() is not protected by task_lock(), so there's a race:

	select_bad_process()
	oom_kill_process(p)
					do_exit()
					exit_signals(p) /* PF_EXITING */
	oom_kill_task(p)
	__oom_kill_task(p)
					exit_mm(p)
					task_lock(p)
					p->mm = NULL
					task_unlock(p)
	printk() of p->mm->total_vm

Please merge this as a fix.
Signed-off-by: David Rientjes <rientjes@google.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/oom_kill.c

@@ -367,14 +367,16 @@ static void __oom_kill_task(struct task_struct *p, int verbose)
 		return;
 	}
 
+	task_lock(p);
 	if (!p->mm) {
 		WARN_ON(1);
-		printk(KERN_WARNING "tried to kill an mm-less task!\n");
+		printk(KERN_WARNING "tried to kill an mm-less task %d (%s)!\n",
+			task_pid_nr(p), p->comm);
+		task_unlock(p);
 		return;
 	}
 
-	if (verbose) {
-		task_lock(p);
+	if (verbose)
 		printk(KERN_ERR "Killed process %d (%s) "
 		       "vsz:%lukB, anon-rss:%lukB, file-rss:%lukB\n",
 		       task_pid_nr(p), p->comm,
@@ -382,7 +384,6 @@ static void __oom_kill_task(struct task_struct *p, int verbose)
 		       K(get_mm_counter(p->mm, anon_rss)),
 		       K(get_mm_counter(p->mm, file_rss)));
 	task_unlock(p);
-	}
 
 	/*
 	 * We give our sacrificial lamb high priority and access to