memcg: clear pc->mem_cgroup if necessary.

This is a preparation before removing a flag PCG_ACCT_LRU in page_cgroup
and reducing atomic ops/complexity in memcg LRU handling.

In some cases, pages are added to lru before charge to memcg and pages
are not classfied to memory cgroup at lru addtion. Now, the lru where
the page should be added is determined a bit in page_cgroup->flags and
pc->mem_cgroup. I'd like to remove the check of flag.

To handle the case pc->mem_cgroup may contain stale pointers if pages
are added to LRU before classification. This patch resets
pc->mem_cgroup to root_mem_cgroup before lru additions.

[[e-mail]: fix CONFIG_CGROUP_MEM_CONT=n build]
[[e-mail]: fix CONFIG_CGROUP_MEM_RES_CTLR=y CONFIG_CGROUP_MEM_RES_CTLR_SWAP=n build]
[[e-mail]: ksm.c needs memcontrol.h, per Michal]
[[e-mail]: stop oops in mem_cgroup_reset_owner()]
[[e-mail]: fix page migration to reset_owner]
Signed-off-by: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Miklos Szeredi <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Ying Han <[e-mail]>
Signed-off-by: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/memcontrol.h

···
 extern void mem_cgroup_replace_page_cache(struct page *oldpage,
 					struct page *newpage);
 
+extern void mem_cgroup_reset_owner(struct page *page);
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 extern int do_swap_account;
 #endif
···
 }
 static inline void mem_cgroup_replace_page_cache(struct page *oldpage,
 				struct page *newpage)
+{
+}
+
+static inline void mem_cgroup_reset_owner(struct page *page)
 {
 }
 #endif /* CONFIG_CGROUP_MEM_CONT */

mm/ksm.c

···
 #include <linux/kthread.h>
 #include <linux/wait.h>
 #include <linux/slab.h>
+#include <linux/memcontrol.h>
 #include <linux/rbtree.h>
 #include <linux/memory.h>
 #include <linux/mmu_notifier.h>
···
 
 	new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
 	if (new_page) {
+		/*
+		 * The memcg-specific accounting when moving
+		 * pages around the LRU lists relies on the
+		 * page's owner (memcg) to be valid.  Usually,
+		 * pages are assigned to a new owner before
+		 * being put on the LRU list, but since this
+		 * is not the case here, the stale owner from
+		 * a previous allocation cycle must be reset.
+		 */
+		mem_cgroup_reset_owner(new_page);
 		copy_user_highpage(new_page, page, address, vma);
 
 		SetPageDirty(new_page);

mm/memcontrol.c

···
 	batch->memcg = NULL;
 }
 
+/*
+ * A function for resetting pc->mem_cgroup for newly allocated pages.
+ * This function should be called if the newpage will be added to LRU
+ * before start accounting.
+ */
+void mem_cgroup_reset_owner(struct page *newpage)
+{
+	struct page_cgroup *pc;
+
+	if (mem_cgroup_disabled())
+		return;
+
+	pc = lookup_page_cgroup(newpage);
+	VM_BUG_ON(PageCgroupUsed(pc));
+	pc->mem_cgroup = root_mem_cgroup;
+}
+
 #ifdef CONFIG_SWAP
 /*
  * called after __delete_from_swap_cache() and drop "page" account.

mm/migrate.c

···
 	if (!newpage)
 		return -ENOMEM;
 
+	mem_cgroup_reset_owner(newpage);
+
 	if (page_count(page) == 1) {
 		/* page was freed from under us. So we are done. */
 		goto out;

mm/swap_state.c

···
 			new_page = alloc_page_vma(gfp_mask, vma, addr);
 			if (!new_page)
 				break;		/* Out of memory */
+			/*
+			 * The memcg-specific accounting when moving
+			 * pages around the LRU lists relies on the
+			 * page's owner (memcg) to be valid.  Usually,
+			 * pages are assigned to a new owner before
+			 * being put on the LRU list, but since this
+			 * is not the case here, the stale owner from
+			 * a previous allocation cycle must be reset.
+			 */
+			mem_cgroup_reset_owner(new_page);
 		}
 
 		/*