[PATCH-next] mm/memcontrol.c: Fix potential uninitialized variable warning

[Waiman Long]

If the -Wno-maybe-uninitialized gcc option is not specified, compilation
of memcontrol.c may generate the following warnings:

mm/memcontrol.c: In function ‘refill_obj_stock’:
./arch/x86/include/asm/irqflags.h:127:17: warning: ‘flags’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  return !(flags & X86_EFLAGS_IF);
          ~~~~~~~^~~~~~~~~~~~~~~~
mm/memcontrol.c:3216:16: note: ‘flags’ was declared here
  unsigned long flags;
                ^~~~~
In file included from mm/memcontrol.c:29:
mm/memcontrol.c: In function ‘uncharge_page’:
./include/linux/memcontrol.h:797:2: warning: ‘objcg’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  percpu_ref_put(&objcg->refcnt);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix that by properly initializing *pflags in get_obj_stock() and
introducing a use_objcg bool variable in uncharge_page() to avoid
potentially accessing the struct page data twice.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 mm/memcontrol.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cb864f87b01d..b9a6db6a7d4f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2108,6 +2108,7 @@ static inline struct obj_stock *get_obj_stock(unsigned long *pflags)
 	struct memcg_stock_pcp *stock;
 
 	if (likely(in_task())) {
+		*pflags = 0UL;
 		preempt_disable();
 		stock = this_cpu_ptr(&memcg_stock);
 		return &stock->task_obj;
@@ -6840,6 +6841,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
 	unsigned long nr_pages;
 	struct mem_cgroup *memcg;
 	struct obj_cgroup *objcg;
+	bool use_objcg = PageMemcgKmem(page);
 
 	VM_BUG_ON_PAGE(PageLRU(page), page);
 
@@ -6848,7 +6850,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
 	 * page memcg or objcg at this point, we have fully
 	 * exclusive access to the page.
 	 */
-	if (PageMemcgKmem(page)) {
+	if (use_objcg) {
 		objcg = __page_objcg(page);
 		/*
 		 * This get matches the put at the end of the function and
@@ -6876,7 +6878,7 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
 
 	nr_pages = compound_nr(page);
 
-	if (PageMemcgKmem(page)) {
+	if (use_objcg) {
 		ug->nr_memory += nr_pages;
 		ug->nr_kmem += nr_pages;
 
-- 
2.18.1