Re: [PATCH] exit: fix oops in sync_mm_rss

[KAMEZAWA Hiroyuki]

On Tue, 30 Mar 2010 18:22:58 -0400
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Wed, 31 Mar 2010 09:41:24 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> 
> > > With this fixed, the test for non-zero tsk->mm is't really needed in
> > > do_exit(), is it?  I guess it makes sense though - sync_mm_rss() only
> > > really works for kernel threads by luck..
> > 
> > At first, I considered so, too. But I changed my mind to show
> > "we know tsk->mm can be NULL here!" by code. 
> > Because __sync_mm_rss_stat() has BUG_ON(!mm), the code reader will think
> > tsk->mm shouldn't be NULL always.
> > 
> > Doesn't make sense ?
> 
> uh, not really ;)
> 
> 
> I think we should do this too:
> 
> --- a/mm/memory.c~exit-fix-oops-in-sync_mm_rss-fix
> +++ a/mm/memory.c
> @@ -131,7 +131,6 @@ static void __sync_task_rss_stat(struct 
>  
>  	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;
>  		}
> _
> 
> Because we just made sure it can't happen, and if it _does_ happen, the
> oops will tell us the samme thing that the BUG_ON() would have.
> 

Hmm, then, finaly..
==

task->rss_stat wasn't initialized to 0 at copy_process().
And __sync_task_rss_stat() should be static.
removed BUG_ON(!mm) in __sync_task_rss_stat() for avoiding to show
wrong information to code readers. Anyway, if !mm && task->rss_stat
has some value, panic will happen.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 kernel/fork.c |    3 +++
 mm/memory.c   |    3 +--
 2 files changed, 4 insertions(+), 2 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;
 







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