Commit dbd4ea78 authored by KAMEZAWA Hiroyuki's avatar KAMEZAWA Hiroyuki Committed by Linus Torvalds

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

  * 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

Performance Impact: moving a 8G anon process.

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

	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: default avatarKAMEZAWA Hiroyuki <>
Signed-off-by: default avatarGreg Thelen <>
Reviewed-by: default avatarMinchan Kim <>
Acked-by: default avatarDaisuke Nishimura <>
Cc: Andrea Righi <>
Cc: Balbir Singh <>
Cc: Wu Fengguang <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 2a7106f2
......@@ -35,15 +35,18 @@ struct page_cgroup *lookup_page_cgroup(struct page *page);
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) \
......@@ -117,6 +120,10 @@ static inline enum zone_type page_cgroup_zid(struct page_cgroup *pc)
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);
......@@ -130,6 +137,24 @@ 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.
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);
struct page_cgroup;
......@@ -1606,6 +1606,7 @@ void mem_cgroup_update_page_stat(struct page *page,
struct mem_cgroup *mem;
struct page_cgroup *pc = lookup_page_cgroup(page);
bool need_unlock = false;
unsigned long uninitialized_var(flags);
if (unlikely(!pc))
......@@ -1617,7 +1618,7 @@ void mem_cgroup_update_page_stat(struct page *page,
/* pc->mem_cgroup is unstable ? */
if (unlikely(mem_cgroup_stealed(mem))) {
/* take a lock against to access pc->mem_cgroup */
move_lock_page_cgroup(pc, &flags);
need_unlock = true;
mem = pc->mem_cgroup;
if (!mem || !PageCgroupUsed(pc))
......@@ -1640,7 +1641,7 @@ void mem_cgroup_update_page_stat(struct page *page,
if (unlikely(need_unlock))
move_unlock_page_cgroup(pc, &flags);
......@@ -2211,9 +2212,13 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)
int ret = -EINVAL;
unsigned long flags;
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;
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