mm: avoid null-pointer deref in sync_mm_rss()

- We weren't zeroing p->rss_stat[] at fork()

- Consequently sync_mm_rss() was dereferencing tsk->mm for kernel
  threads and was oopsing.

- Make __sync_task_rss_stat() static, too.

Addresses https://bugzilla.kernel.org/show_bug.cgi?id=15648

[akpm@linux-foundation.org: remove the BUG_ON(!mm->rss)]
Reported-by: Troels Liebe Bentsen <tlb@rapanden.dk>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
"Michael S. Tsirkin" <mst@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -953,6 +953,7 @@ NORET_TYPE void do_exit(long code)
 
 	acct_update_integrals(tsk);
 	/* sync mm's RSS info before statistics gathering */
+	if (tsk->mm)
 		sync_mm_rss(tsk, tsk->mm);
 	group_dead = atomic_dec_and_test(&tsk->signal->live);
 	if (group_dead) {

kernel/fork.c

@@ -1052,6 +1052,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	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;
 

mm/memory.c

@@ -125,13 +125,12 @@ 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;
 		}