memcg: add lock to synchronize page accounting and migration

Introduce a new bit spin lock, PCG_MOVE_LOCK, to synchronize the page
accounting and migration code. This reworks the locking scheme of
_update_stat() and _move_account() by adding new lock bit PCG_MOVE_LOCK,
which is always taken under IRQ disable.

1. If pages are being migrated from a memcg, then updates to that
memcg page statistics are protected by grabbing PCG_MOVE_LOCK using
move_lock_page_cgroup(). In an upcoming commit, memcg dirty page
accounting will be updating memcg page accounting (specifically: num
writeback pages) from IRQ context (softirq). Avoid a deadlocking
nested spin lock attempt by disabling irq on the local processor when
grabbing the PCG_MOVE_LOCK.

2. lock for update_page_stat is used only for avoiding race with
move_account(). So, IRQ awareness of lock_page_cgroup() itself is not
a problem. The problem is between mem_cgroup_update_page_stat() and
mem_cgroup_move_account_page().

Trade-off:
* Changing lock_page_cgroup() to always disable IRQ (or
local_bh) has some impacts on performance and I think
it's bad to disable IRQ when it's not necessary.
* adding a new lock makes move_account() slower. Score is
here.

Performance Impact: moving a 8G anon process.

Before:
real 0m0.792s
user 0m0.000s
sys 0m0.780s

After:
real 0m0.854s
user 0m0.000s
sys 0m0.842s

This score is bad but planned patches for optimization can reduce
this impact.

Signed-off-by: KAMEZAWA Hiroyuki <[e-mail]>
Signed-off-by: Greg Thelen <[e-mail]>
Reviewed-by: Minchan Kim <[e-mail]>
Acked-by: Daisuke Nishimura <[e-mail]>
Cc: Andrea Righi <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Cc: Wu Fengguang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/page_cgroup.h

···
 
 enum {
 	/* flags for mem_cgroup */
-	PCG_LOCK,  /* page cgroup is locked */
+	PCG_LOCK,  /* Lock for pc->mem_cgroup and following bits. */
 	PCG_CACHE, /* charged as cache */
 	PCG_USED, /* this object is in use. */
-	PCG_ACCT_LRU, /* page has been accounted for */
+	PCG_MIGRATION, /* under page migration */
+	/* flags for mem_cgroup and file and I/O status */
+	PCG_MOVE_LOCK, /* For race between move_account v.s. following bits */
 	PCG_FILE_MAPPED, /* page is accounted as "mapped" */
 	PCG_FILE_DIRTY, /* page is dirty */
 	PCG_FILE_WRITEBACK, /* page is under writeback */
 	PCG_FILE_UNSTABLE_NFS, /* page is NFS unstable */
-	PCG_MIGRATION, /* under page migration */
+	/* No lock in page_cgroup */
+	PCG_ACCT_LRU, /* page has been accounted for (under lru_lock) */
 };
 
 #define TESTPCGFLAG(uname, lname)			\
···
 
 static inline void lock_page_cgroup(struct page_cgroup *pc)
 {
+	/*
+	 * Don't take this lock in IRQ context.
+	 * This lock is for pc->mem_cgroup, USED, CACHE, MIGRATION
+	 */
 	bit_spin_lock(PCG_LOCK, &pc->flags);
 }
 
···
 static inline int page_is_cgroup_locked(struct page_cgroup *pc)
 {
 	return bit_spin_is_locked(PCG_LOCK, &pc->flags);
+}
+
+static inline void move_lock_page_cgroup(struct page_cgroup *pc,
+	unsigned long *flags)
+{
+	/*
+	 * We know updates to pc->flags of page cache's stats are from both of
+	 * usual context or IRQ context. Disable IRQ to avoid deadlock.
+	 */
+	local_irq_save(*flags);
+	bit_spin_lock(PCG_MOVE_LOCK, &pc->flags);
+}
+
+static inline void move_unlock_page_cgroup(struct page_cgroup *pc,
+	unsigned long *flags)
+{
+	bit_spin_unlock(PCG_MOVE_LOCK, &pc->flags);
+	local_irq_restore(*flags);
 }
 
 #else /* CONFIG_CGROUP_MEM_RES_CTLR */

mm/memcontrol.c

···
 	struct mem_cgroup *mem;
 	struct page_cgroup *pc = lookup_page_cgroup(page);
 	bool need_unlock = false;
+	unsigned long uninitialized_var(flags);
 
 	if (unlikely(!pc))
 		return;
···
 	/* pc->mem_cgroup is unstable ? */
 	if (unlikely(mem_cgroup_stealed(mem))) {
 		/* take a lock against to access pc->mem_cgroup */
-		lock_page_cgroup(pc);
+		move_lock_page_cgroup(pc, &flags);
 		need_unlock = true;
 		mem = pc->mem_cgroup;
 		if (!mem || !PageCgroupUsed(pc))
···
 
 out:
 	if (unlikely(need_unlock))
-		unlock_page_cgroup(pc);
+		move_unlock_page_cgroup(pc, &flags);
 	rcu_read_unlock();
 	return;
 }
···
 		struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)
 {
 	int ret = -EINVAL;
+	unsigned long flags;
+
 	lock_page_cgroup(pc);
 	if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {
+		move_lock_page_cgroup(pc, &flags);
 		__mem_cgroup_move_account(pc, from, to, uncharge);
+		move_unlock_page_cgroup(pc, &flags);
 		ret = 0;
 	}
 	unlock_page_cgroup(pc);