mm: memcontrol: avoid charge statistics churn during page migration

Charge migration currently disables IRQs twice to update the charge
statistics for the old page and then again for the new page.

But migration is a seamless transition of a charge from one physical
page to another one of the same size, so this should be a non-event from
an accounting point of view. Leave the statistics alone.

Signed-off-by: Johannes Weiner <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 }
 
 static void commit_charge(struct page *page, struct mem_cgroup *memcg,
-			  unsigned int nr_pages, bool lrucare)
+			  bool lrucare)
 {
 	struct page_cgroup *pc = lookup_page_cgroup(page);
 	int isolated;
···
 
 	if (lrucare)
 		unlock_page_lru(page, isolated);
-
-	local_irq_disable();
-	mem_cgroup_charge_statistics(memcg, page, nr_pages);
-	/*
-	 * "charge_statistics" updated event counter. Then, check it.
-	 * Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
-	 * if they exceeds softlimit.
-	 */
-	memcg_check_events(memcg, page);
-	local_irq_enable();
 }
 
 static DEFINE_MUTEX(set_limit_mutex);
···
 	if (!memcg)
 		return;
 
+	commit_charge(page, memcg, lrucare);
+
 	if (PageTransHuge(page)) {
 		nr_pages <<= compound_order(page);
 		VM_BUG_ON_PAGE(!PageTransHuge(page), page);
 	}
 
-	commit_charge(page, memcg, nr_pages, lrucare);
+	local_irq_disable();
+	mem_cgroup_charge_statistics(memcg, page, nr_pages);
+	memcg_check_events(memcg, page);
+	local_irq_enable();
 
 	if (do_swap_account && PageSwapCache(page)) {
 		swp_entry_t entry = { .val = page_private(page) };
···
 void mem_cgroup_migrate(struct page *oldpage, struct page *newpage,
 			bool lrucare)
 {
-	unsigned int nr_pages = 1;
 	struct page_cgroup *pc;
 	int isolated;
 
···
 	VM_BUG_ON_PAGE(!lrucare && PageLRU(oldpage), oldpage);
 	VM_BUG_ON_PAGE(!lrucare && PageLRU(newpage), newpage);
 	VM_BUG_ON_PAGE(PageAnon(oldpage) != PageAnon(newpage), newpage);
+	VM_BUG_ON_PAGE(PageTransHuge(oldpage) != PageTransHuge(newpage),
+		       newpage);
 
 	if (mem_cgroup_disabled())
 		return;
···
 	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEM), oldpage);
 	VM_BUG_ON_PAGE(do_swap_account && !(pc->flags & PCG_MEMSW), oldpage);
 
-	if (PageTransHuge(oldpage)) {
-		nr_pages <<= compound_order(oldpage);
-		VM_BUG_ON_PAGE(!PageTransHuge(oldpage), oldpage);
-		VM_BUG_ON_PAGE(!PageTransHuge(newpage), newpage);
-	}
-
 	if (lrucare)
 		lock_page_lru(oldpage, &isolated);
 
···
 	if (lrucare)
 		unlock_page_lru(oldpage, isolated);
 
-	local_irq_disable();
-	mem_cgroup_charge_statistics(pc->mem_cgroup, oldpage, -nr_pages);
-	memcg_check_events(pc->mem_cgroup, oldpage);
-	local_irq_enable();
-
-	commit_charge(newpage, pc->mem_cgroup, nr_pages, lrucare);
+	commit_charge(newpage, pc->mem_cgroup, lrucare);
 }
 
 /*