linux-kernel - Re: [PATCH] exit: fix oops in sync_mm

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Date:	Wed, 31 Mar 2010 12:11:20 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	"Michael S. Tsirkin" <mst@...hat.com>, cl@...ux-foundation.org,
	lee.schermerhorn@...com, rientjes@...gle.com,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Troels Liebe Bentsen <tlb@...anden.dk>,
	linux-bluetooth@...r.kernel.org
Subject: Re: [PATCH] exit: fix oops in sync_mm_rss

On Tue, 30 Mar 2010 20:03:36 -0400
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 31 Mar 2010 11:53:00 +0900 Minchan Kim <minchan.kim@...il.com> wrote:
 
> really.  Apart from the fact that we'll otherwise perform an empty
> NR_MM_COUNTERS loop in __sync_task_rss_stat(), sync_mm_rss() just isn't
> set up to handle kernel threads.  Given that the function of
> sync_task_mm(from, to) is to move stuff from "from" and into "to", it's
> daft to call it with a NULL value of `to'!
> 
Updated again.

==

task->rss_stat wasn't initialized to 0 at copy_process().
At exit, tsk->mm may be NULL. It's not valid to call sync_mm_rss()
against not exisiting mm_struct, We should check it.
And __sync_task_rss_stat() should be static.
This patch also removes BUG_ON(!mm) in __sync_task_rss_stat().
The code will panic if !mm without it.

Changelog:
 - removed BUG_ON()
 - added check task->mm in exit.c before calling sync_mm_rss().

Reviewed-by: Minchan Kim <minchan.kim@...il.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
---
 kernel/exit.c |    3 ++-
 kernel/fork.c |    3 +++
 mm/memory.c   |    3 +--
 3 files changed, 6 insertions(+), 3 deletions(-)

Index: mmotm-2.6.34-Mar24/mm/memory.c
===================================================================
--- mmotm-2.6.34-Mar24.orig/mm/memory.c
+++ mmotm-2.6.34-Mar24/mm/memory.c
@@ -124,13 +124,12 @@ core_initcall(init_zero_pfn);
 
 #if defined(SPLIT_RSS_COUNTING)
 
-void __sync_task_rss_stat(struct task_struct *task, struct mm_struct *mm)
+static void __sync_task_rss_stat(struct task_struct *task, struct mm_struct *mm)
 {
 	int i;
 
 	for (i = 0; i < NR_MM_COUNTERS; i++) {
 		if (task->rss_stat.count[i]) {
-			BUG_ON(!mm);
 			add_mm_counter(mm, i, task->rss_stat.count[i]);
 			task->rss_stat.count[i] = 0;
 		}
Index: mmotm-2.6.34-Mar24/kernel/fork.c
===================================================================
--- mmotm-2.6.34-Mar24.orig/kernel/fork.c
+++ mmotm-2.6.34-Mar24/kernel/fork.c
@@ -1060,6 +1060,9 @@ static struct task_struct *copy_process(
 	p->prev_utime = cputime_zero;
 	p->prev_stime = cputime_zero;
 #endif
+#if defined(SPLIT_RSS_COUNTING)
+	memset(&p->rss_stat, 0, sizeof(p->rss_stat));
+#endif
 
 	p->default_timer_slack_ns = current->timer_slack_ns;
 
Index: mmotm-2.6.34-Mar24/kernel/exit.c
===================================================================
--- mmotm-2.6.34-Mar24.orig/kernel/exit.c
+++ mmotm-2.6.34-Mar24/kernel/exit.c
@@ -950,7 +950,8 @@ NORET_TYPE void do_exit(long code)
 
 	acct_update_integrals(tsk);
 	/* sync mm's RSS info before statistics gathering */
-	sync_mm_rss(tsk, tsk->mm);
+	if (tsk->mm)
+		sync_mm_rss(tsk, tsk->mm);
 	group_dead = atomic_dec_and_test(&tsk->signal->live);
 	if (group_dead) {
 		hrtimer_cancel(&tsk->signal->real_timer);

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