mm: memcontrol: do not uncharge old page in page cache replacement

Changing page->mem_cgroup of a live page is tricky and fragile.  In
particular, the memcg writeback code relies on that mapping being stable
and users of mem_cgroup_replace_page() not overlapping with dirtyable
inodes.

Page cache replacement doesn't have to do that, though.  Instead of being
clever and transferring the charge from the old page to the new,
force-charge the new page and leave the old page alone.  A temporary
overcharge won't matter in practice, and the old page is going to be freed
shortly after this anyway.  And this is not performance critical.
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -366,13 +366,6 @@ mem_cgroup_zone_zoneinfo(struct mem_cgroup *memcg, struct zone *zone)
  *
  * If memcg is bound to a traditional hierarchy, the css of root_mem_cgroup
  * is returned.
- *
- * XXX: The above description of behavior on the default hierarchy isn't
- * strictly true yet as replace_page_cache_page() can modify the
- * association before @page is released even on the default hierarchy;
- * however, the current and planned usages don't mix the the two functions
- * and replace_page_cache_page() will soon be updated to make the invariant
- * actually true.
  */
 struct cgroup_subsys_state *mem_cgroup_css_from_page(struct page *page)
 {
@@ -5464,7 +5457,8 @@ void mem_cgroup_uncharge_list(struct list_head *page_list)
 void mem_cgroup_replace_page(struct page *oldpage, struct page *newpage)
 {
 	struct mem_cgroup *memcg;
-	int isolated;
+	unsigned int nr_pages;
+	bool compound;
 
 	VM_BUG_ON_PAGE(!PageLocked(oldpage), oldpage);
 	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
@@ -5484,11 +5478,21 @@ void mem_cgroup_replace_page(struct page *oldpage, struct page *newpage)
 	if (!memcg)
 		return;
 
-	lock_page_lru(oldpage, &isolated);
-	oldpage->mem_cgroup = NULL;
-	unlock_page_lru(oldpage, isolated);
+	/* Force-charge the new page. The old one will be freed soon */
+	compound = PageTransHuge(newpage);
+	nr_pages = compound ? hpage_nr_pages(newpage) : 1;
+
+	page_counter_charge(&memcg->memory, nr_pages);
+	if (do_memsw_account())
+		page_counter_charge(&memcg->memsw, nr_pages);
+	css_get_many(&memcg->css, nr_pages);
 
 	commit_charge(newpage, memcg, true);
+
+	local_irq_disable();
+	mem_cgroup_charge_statistics(memcg, newpage, compound, nr_pages);
+	memcg_check_events(memcg, newpage);
+	local_irq_enable();
 }
 
 DEFINE_STATIC_KEY_FALSE(memcg_sockets_enabled_key);