[RFC][PATCH 3/6] memcg: add PageCgroupReset()

[KAMEZAWA Hiroyuki]

 A commit "memcg: simplify LRU handling by new rule" removes PCG_ACCT_LRU.
 and the bug introduced by it was fixed by "memcg: fix GPF when cgroup removal
 races with last exit"

This was for reducing flags on pc->flags....Now, we have 3bits of flags.
but this patch adds a new flag, I'm sorry. (Considering alignment of
kmalloc(), we'll able to have 5 bits..)

This patch adds PCG_RESET which is similar to PCG_ACCT_LRU. This is set
when mem_cgroup_add_lru_list() finds we cannot trust the pc's mem_cgroup.

The reason why this patch adds a (renamed) flag again is for merging
pc->flags and pc->mem_cgroup. Assume pc's mem_cgroup is encoded as

	mem_cgroup = pc->flags & ~0x7

Updating multiple bits of pc->flags without talking lock_page_cgroup()
is very dangerous. And mem_cgroup_add_lru_list() updates pc->mem_cgroup
without taking lock. Then I add RESET bit. After this, pc_to_mem_cgroup()
is written as

	if (PageCgroupReset(pc))
		return root_mem_cgroup;
	return pc->mem_cgroup;

This update of Reset bit can be done in atomic by set_bit(). And
cleared when USED bit is set.

Considering kmalloc()'s alignment, having 4bits of flags will be ok....

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 include/linux/page_cgroup.h |   15 ++++++++-------
 mm/memcontrol.c             |    5 +++--
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index 2707809..3f3b4ff 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -8,6 +8,7 @@ enum {
 	PCG_LOCK,  /* Lock for pc->mem_cgroup and following bits. */
 	PCG_USED, /* this object is in use. */
 	PCG_MIGRATION, /* under page migration */
+	PCG_RESET,     /* have been reset to root_mem_cgroup */
 	__NR_PCG_FLAGS,
 };
 
@@ -70,6 +71,9 @@ SETPCGFLAG(Migration, MIGRATION)
 CLEARPCGFLAG(Migration, MIGRATION)
 TESTPCGFLAG(Migration, MIGRATION)
 
+TESTPCGFLAG(Reset, RESET)
+SETPCGFLAG(Reset, RESET)
+
 static inline void lock_page_cgroup(struct page_cgroup *pc)
 {
 	/*
@@ -84,16 +88,13 @@ static inline void unlock_page_cgroup(struct page_cgroup *pc)
 	bit_spin_unlock(PCG_LOCK, &pc->flags);
 }
 
+extern struct mem_cgroup*  root_mem_cgroup;
 
 static inline struct mem_cgroup* pc_to_mem_cgroup(struct page_cgroup *pc)
 {
-	return pc->mem_cgroup;
-}
-
-static inline void
-pc_set_mem_cgroup(struct page_cgroup *pc, struct mem_cgroup *memcg)
-{
-	pc->mem_cgroup = memcg;
+	if (likely(!PageCgroupReset(pc)))
+		return pc->mem_cgroup;
+	return root_mem_cgroup;
 }
 
 static inline void
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d366b60..622fd2e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1080,7 +1080,8 @@ struct lruvec *mem_cgroup_lru_add_list(struct zone *zone, struct page *page,
 	 * of pc's mem_cgroup safe.
 	 */
 	if (!PageCgroupUsed(pc) && memcg != root_mem_cgroup) {
-		pc_set_mem_cgroup(pc, root_mem_cgroup);
+		/* this reset bit is cleared when the page is charged */
+		SetPageCgroupReset(pc);
 		memcg = root_mem_cgroup;
 	}
 
@@ -2626,7 +2627,7 @@ static int mem_cgroup_move_account(struct page *page,
 		__mem_cgroup_cancel_charge(from, nr_pages);
 
 	/* caller should have done css_get */
-	pc_set_mem_cgroup(pc, to);
+	pc_set_mem_cgroup_and_flags(pc, to, BIT(PCG_USED) | BIT(PCG_LOCK));
 	mem_cgroup_charge_statistics(to, anon, nr_pages);
 	/*
 	 * We charges against "to" which may not have any tasks. Then, "to"
-- 
1.7.4.1


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