memcg: fix race at move_parent around compound_order()

A fix up mem_cgroup_move_parent() which use compound_order() in
asynchronous manner. This compound_order() may return unknown value
because we don't take lock. Use PageTransHuge() and HPAGE_SIZE instead
of it.

Also clean up for mem_cgroup_move_parent().
- remove unnecessary initialization of local variable.
- rename charge_size -> page_size
- remove unnecessary (wrong) comment.
- added a comment about THP.

Note:
Current design take compound_page_lock() in caller of move_account().
This should be revisited when we implement direct move_task of hugepage
without splitting.

[[e-mail]: coding-style fixes]
Signed-off-by: KAMEZAWA Hiroyuki <[e-mail]>
Reviewed-by: Johannes Weiner <[e-mail]>
Acked-by: Daisuke Nishimura <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 {
 	int ret = -EINVAL;
 	unsigned long flags;
-
+	/*
+	 * The page is isolated from LRU. So, collapse function
+	 * will not handle this page. But page splitting can happen.
+	 * Do this check under compound_page_lock(). The caller should
+	 * hold it.
+	 */
 	if ((charge_size > PAGE_SIZE) && !PageTransHuge(pc->page))
 		return -EBUSY;
 
···
 	struct cgroup *cg = child->css.cgroup;
 	struct cgroup *pcg = cg->parent;
 	struct mem_cgroup *parent;
-	int charge = PAGE_SIZE;
+	int page_size = PAGE_SIZE;
 	unsigned long flags;
 	int ret;
 
···
 		goto out;
 	if (isolate_lru_page(page))
 		goto put;
-	/* The page is isolated from LRU and we have no race with splitting */
-	charge = PAGE_SIZE << compound_order(page);
+
+	if (PageTransHuge(page))
+		page_size = HPAGE_SIZE;
 
 	parent = mem_cgroup_from_cont(pcg);
-	ret = __mem_cgroup_try_charge(NULL, gfp_mask, &parent, false, charge);
+	ret = __mem_cgroup_try_charge(NULL, gfp_mask,
+				&parent, false, page_size);
 	if (ret || !parent)
 		goto put_back;
 
-	if (charge > PAGE_SIZE)
+	if (page_size > PAGE_SIZE)
 		flags = compound_lock_irqsave(page);
 
-	ret = mem_cgroup_move_account(pc, child, parent, true, charge);
+	ret = mem_cgroup_move_account(pc, child, parent, true, page_size);
 	if (ret)
-		mem_cgroup_cancel_charge(parent, charge);
+		mem_cgroup_cancel_charge(parent, page_size);
 
-	if (charge > PAGE_SIZE)
+	if (page_size > PAGE_SIZE)
 		compound_unlock_irqrestore(page, flags);
 put_back:
 	putback_lru_page(page);