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>

1 files changed, 1 insertions, 2 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 1d2ea39260e5..833952d8b74d 100644
--- a/mm/memory.c
+++ b/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;
                 }