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>

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())

mm/memory.c

@@ -2767,7 +2767,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	swp_entry_t entry;
 	pte_t pte;
 	int locked;
-	struct mem_cgroup *ptr = NULL;
+	struct mem_cgroup *ptr;
 	int exclusive = 0;
 	int ret = 0;
 

mm/migrate.c

@@ -633,7 +633,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
 	struct page *newpage = get_new_page(page, private, &result);
 	int remap_swapcache = 1;
 	int charge = 0;
-	struct mem_cgroup *mem = NULL;
+	struct mem_cgroup *mem;
 	struct anon_vma *anon_vma = NULL;
 
 	if (!newpage)

mm/swapfile.c

@@ -880,7 +880,7 @@ unsigned int count_swap_pages(int type, int free)
 static int unuse_pte(struct vm_area_struct *vma, pmd_t *pmd,
 		unsigned long addr, swp_entry_t entry, struct page *page)
 {
-	struct mem_cgroup *ptr = NULL;
+	struct mem_cgroup *ptr;
 	spinlock_t *ptl;
 	pte_t *pte;
 	int ret = 1;