memcg: simplify corner case handling of LRU.

This patch simplifies LRU handling of racy case (memcg+SwapCache).  At
charging, SwapCache tend to be on LRU already.  So, before overwriting
pc->mem_cgroup, the page must be removed from LRU and added to LRU
later.

This patch does
        spin_lock(zone->lru_lock);
        if (PageLRU(page))
                remove from LRU
        overwrite pc->mem_cgroup
        if (PageLRU(page))
                add to new LRU.
        spin_unlock(zone->lru_lock);

And guarantee all pages are not on LRU at modifying pc->mem_cgroup.
This patch also unfies lru handling of replace_page_cache() and
swapin.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ying Han <yinghan@google.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1136,86 +1136,6 @@ struct lruvec *mem_cgroup_lru_move_lists(struct zone *zone,
 	return mem_cgroup_lru_add_list(zone, page, to);
 }
 
-/*
- * At handling SwapCache and other FUSE stuff, pc->mem_cgroup may be changed
- * while it's linked to lru because the page may be reused after it's fully
- * uncharged. To handle that, unlink page_cgroup from LRU when charge it again.
- * It's done under lock_page and expected that zone->lru_lock isnever held.
- */
-static void mem_cgroup_lru_del_before_commit(struct page *page)
-{
-	enum lru_list lru;
-	unsigned long flags;
-	struct zone *zone = page_zone(page);
-	struct page_cgroup *pc = lookup_page_cgroup(page);
-
-	/*
-	 * Doing this check without taking ->lru_lock seems wrong but this
-	 * is safe. Because if page_cgroup's USED bit is unset, the page
-	 * will not be added to any memcg's LRU. If page_cgroup's USED bit is
-	 * set, the commit after this will fail, anyway.
-	 * This all charge/uncharge is done under some mutual execustion.
-	 * So, we don't need to taking care of changes in USED bit.
-	 */
-	if (likely(!PageLRU(page)))
-		return;
-
-	spin_lock_irqsave(&zone->lru_lock, flags);
-	lru = page_lru(page);
-	/*
-	 * The uncharged page could still be registered to the LRU of
-	 * the stale pc->mem_cgroup.
-	 *
-	 * As pc->mem_cgroup is about to get overwritten, the old LRU
-	 * accounting needs to be taken care of.  Let root_mem_cgroup
-	 * babysit the page until the new memcg is responsible for it.
-	 *
-	 * The PCG_USED bit is guarded by lock_page() as the page is
-	 * swapcache/pagecache.
-	 */
-	if (PageLRU(page) && PageCgroupAcctLRU(pc) && !PageCgroupUsed(pc)) {
-		del_page_from_lru_list(zone, page, lru);
-		add_page_to_lru_list(zone, page, lru);
-	}
-	spin_unlock_irqrestore(&zone->lru_lock, flags);
-}
-
-static void mem_cgroup_lru_add_after_commit(struct page *page)
-{
-	enum lru_list lru;
-	unsigned long flags;
-	struct zone *zone = page_zone(page);
-	struct page_cgroup *pc = lookup_page_cgroup(page);
-	/*
-	 * putback:				charge:
-	 * SetPageLRU				SetPageCgroupUsed
-	 * smp_mb				smp_mb
-	 * PageCgroupUsed && add to memcg LRU	PageLRU && add to memcg LRU
-	 *
-	 * Ensure that one of the two sides adds the page to the memcg
-	 * LRU during a race.
-	 */
-	smp_mb();
-	/* taking care of that the page is added to LRU while we commit it */
-	if (likely(!PageLRU(page)))
-		return;
-	spin_lock_irqsave(&zone->lru_lock, flags);
-	lru = page_lru(page);
-	/*
-	 * If the page is not on the LRU, someone will soon put it
-	 * there.  If it is, and also already accounted for on the
-	 * memcg-side, it must be on the right lruvec as setting
-	 * pc->mem_cgroup and PageCgroupUsed is properly ordered.
-	 * Otherwise, root_mem_cgroup has been babysitting the page
-	 * during the charge.  Move it to the new memcg now.
-	 */
-	if (PageLRU(page) && !PageCgroupAcctLRU(pc)) {
-		del_page_from_lru_list(zone, page, lru);
-		add_page_to_lru_list(zone, page, lru);
-	}
-	spin_unlock_irqrestore(&zone->lru_lock, flags);
-}
-
 /*
  * Checks whether given mem is same or in the root_mem_cgroup's
  * hierarchy subtree
@@ -2775,14 +2695,27 @@ __mem_cgroup_commit_charge_lrucare(struct page *page, struct mem_cgroup *memcg,
 					enum charge_type ctype)
 {
 	struct page_cgroup *pc = lookup_page_cgroup(page);
+	struct zone *zone = page_zone(page);
+	unsigned long flags;
+	bool removed = false;
+
 	/*
 	 * In some case, SwapCache, FUSE(splice_buf->radixtree), the page
 	 * is already on LRU. It means the page may on some other page_cgroup's
 	 * LRU. Take care of it.
 	 */
-	mem_cgroup_lru_del_before_commit(page);
+	spin_lock_irqsave(&zone->lru_lock, flags);
+	if (PageLRU(page)) {
+		del_page_from_lru_list(zone, page, page_lru(page));
+		ClearPageLRU(page);
+		removed = true;
+	}
 	__mem_cgroup_commit_charge(memcg, page, 1, pc, ctype);
-	mem_cgroup_lru_add_after_commit(page);
+	if (removed) {
+		add_page_to_lru_list(zone, page, page_lru(page));
+		SetPageLRU(page);
+	}
+	spin_unlock_irqrestore(&zone->lru_lock, flags);
 	return;
 }
 
@@ -3383,9 +3316,7 @@ void mem_cgroup_replace_page_cache(struct page *oldpage,
 {
 	struct mem_cgroup *memcg;
 	struct page_cgroup *pc;
-	struct zone *zone;
 	enum charge_type type = MEM_CGROUP_CHARGE_TYPE_CACHE;
-	unsigned long flags;
 
 	if (mem_cgroup_disabled())
 		return;
@@ -3401,20 +3332,12 @@ void mem_cgroup_replace_page_cache(struct page *oldpage,
 	if (PageSwapBacked(oldpage))
 		type = MEM_CGROUP_CHARGE_TYPE_SHMEM;
 
-	zone = page_zone(newpage);
-	pc = lookup_page_cgroup(newpage);
 	/*
 	 * Even if newpage->mapping was NULL before starting replacement,
 	 * the newpage may be on LRU(or pagevec for LRU) already. We lock
 	 * LRU while we overwrite pc->mem_cgroup.
 	 */
-	spin_lock_irqsave(&zone->lru_lock, flags);
-	if (PageLRU(newpage))
-		del_page_from_lru_list(zone, newpage, page_lru(newpage));
-	__mem_cgroup_commit_charge(memcg, newpage, 1, pc, type);
-	if (PageLRU(newpage))
-		add_page_to_lru_list(zone, newpage, page_lru(newpage));
-	spin_unlock_irqrestore(&zone->lru_lock, flags);
+	__mem_cgroup_commit_charge_lrucare(newpage, memcg, type);
 }
 
 #ifdef CONFIG_DEBUG_VM