memcg: fix ugly initialization of return value is in caller

Remove initialization of vaiable in caller of memory cgroup function.
Actually, it's return value of memcg function but it's initialized in
caller.

Some memory cgroup uses following style to bring the result of start
function to the end function for avoiding races.

   mem_cgroup_start_A(&(*ptr))
   /* Something very complicated can happen here. */
   mem_cgroup_end_A(*ptr)

In some calls, *ptr should be initialized to NULL be caller.  But it's
ugly.  This patch fixes that *ptr is initialized by _start function.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e1ee6ad9c971..b56bd74b486f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2475,7 +2475,7 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
 
         /* shmem */
         if (PageSwapCache(page)) {
-                struct mem_cgroup *mem = NULL;
+                struct mem_cgroup *mem;
 
                 ret = mem_cgroup_try_charge_swapin(mm, page, gfp_mask, &mem);
                 if (!ret)
@@ -2501,6 +2501,8 @@ int mem_cgroup_try_charge_swapin(struct mm_struct *mm,
         struct mem_cgroup *mem;
         int ret;
 
+        *ptr = NULL;
+
         if (mem_cgroup_disabled())
                 return 0;
 
@@ -2916,6 +2918,8 @@ int mem_cgroup_prepare_migration(struct page *page,
         enum charge_type ctype;
         int ret = 0;
 
+        *ptr = NULL;
+
         VM_BUG_ON(PageTransHuge(page));
         if (mem_cgroup_disabled())
                 return 0;
@@ -3058,7 +3062,7 @@ int mem_cgroup_shmem_charge_fallback(struct page *page,
                             struct mm_struct *mm,
                             gfp_t gfp_mask)
 {
-        struct mem_cgroup *mem = NULL;
+        struct mem_cgroup *mem;
         int ret;
 
         if (mem_cgroup_disabled())