[RFC] [PATCH 3/9] memcg: move_account between groups

KAMEZAWA Hiroyuki
2008-09-11T11:12:48Z

This patch provides a function to move account information of a page between
mem_cgroups.

This moving of page_cgroup is done under
- the page is locked.
- lru_lock of source/destination mem_cgroup is held.

Then, a routine which touches pc->mem_cgroup without page_lock() should
confirm pc->mem_cgroup is still valid or not. Typlical code can be following.

(while page is not under lock_page())
mem = pc->mem_cgroup;
mz = page_cgroup_zoneinfo(pc)
spin_lock_irqsave(&mz->lru_lock);
if (pc->mem_cgroup == mem)
...../* some list handling */
spin_unlock_irq(&mz->lru_lock);

If you find page_cgroup from mem_cgroup's LRU under mz->lru_lock, you don't
have to worry about anything.

Changelong: (v2) -> (v3)
- added lock_page_cgroup().
- splitted out from new-force-empty patch.
- added how-to-use text.
- fixed race in __mem_cgroup_uncharge_common().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

---
mm/memcontrol.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 71 insertions(+), 3 deletions(-)

Index: mmtom-2.6.27-rc5+/mm/memcontrol.c
===================================================================
--- mmtom-2.6.27-rc5+.orig/mm/memcontrol.c
+++ mmtom-2.6.27-rc5+/mm/memcontrol.c
@@ -428,6 +428,7 @@ int task_in_mem_cgroup(struct task_struc
void mem_cgroup_move_lists(struct page *page, enum lru_list lru)
{
struct page_cgroup *pc;
+ struct mem_cgroup *mem;
struct mem_cgroup_per_zone *mz;
unsigned long flags;

@@ -446,9 +447,14 @@ void mem_cgroup_move_lists(struct page *

pc = page_get_page_cgroup(page);
if (pc) {
+ mem = pc->mem_cgroup;
mz = page_cgroup_zoneinfo(pc);
spin_lock_irqsave(&mz->lru_lock, flags);
- __mem_cgroup_move_lists(pc, lru);
+ /*
+ * check against the race with move_account.
+ */
+ if (likely(mem == pc->mem_cgroup))
+ __mem_cgroup_move_lists(pc, lru);
spin_unlock_irqrestore(&mz->lru_lock, flags);
}
unlock_page_cgroup(page);
@@ -569,6 +575,67 @@ unsigned long mem_cgroup_isolate_pages(u
return nr_taken;
}

+/**
+ * mem_cgroup_move_account - move account of the page
+ * @page ... the target page of being moved.
+ * @pc ... page_cgroup of the page.
+ * @from ... mem_cgroup which the page is moved from.
+ * @to ... mem_cgroup which the page is moved to.
+ *
+ * The caller must confirm following.
+ * 1. lock the page by lock_page().
+ * 2. disable irq.
+ * 3. lru_lock of old mem_cgroup should be held.
+ * 4. pc is guaranteed to be valid and on mem_cgroup's LRU.
+ *
+ * Because we cannot call try_to_free_page() here, the caller must guarantee
+ * this moving of change never fails. Currently this is called only against
+ * root cgroup, which has no limitation of resource.
+ * Returns 0 at success, returns 1 at failure.
+ */
+int mem_cgroup_move_account(struct page *page, struct page_cgroup *pc,
+ struct mem_cgroup *from, struct mem_cgroup *to)
+{
+ struct mem_cgroup_per_zone *from_mz, *to_mz;
+ int nid, zid;
+ int ret = 1;
+
+ VM_BUG_ON(!irqs_disabled());
+ VM_BUG_ON(!PageLocked(page));
+
+ nid = page_to_nid(page);
+ zid = page_zonenum(page);
+ from_mz = mem_cgroup_zoneinfo(from, nid, zid);
+ to_mz = mem_cgroup_zoneinfo(to, nid, zid);
+
+ if (res_counter_charge(&to->res, PAGE_SIZE)) {
+ /* Now, we assume no_limit...no failure here. */
+ return ret;
+ }
+ if (try_lock_page_cgroup(page))
+ return ret;
+
+ if (page_get_page_cgroup(page) != pc)
+ goto out;
+
+ if (spin_trylock(&to_mz->lru_lock)) {
+ __mem_cgroup_remove_list(from_mz, pc);
+ css_put(&from->css);
+ res_counter_uncharge(&from->res, PAGE_SIZE);
+ pc->mem_cgroup = to;
+ css_get(&to->css);
+ __mem_cgroup_add_list(to_mz, pc);
+ ret = 0;
+ spin_unlock(&to_mz->lru_lock);
+ } else {
+ res_counter_uncharge(&to->res, PAGE_SIZE);
+ }
+out:
+ unlock_page_cgroup(page);
+
+ return ret;
+}
+
/*
* Charge the memory controller for page usage.
* Return
@@ -761,16 +828,24 @@ __mem_cgroup_uncharge_common(struct page
if ((ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
&& ((PageCgroupCache(pc) || page_mapped(page))))
goto unlock;
-
+retry:
+ mem = pc->mem_cgroup;
mz = page_cgroup_zoneinfo(pc);
spin_lock_irqsave(&mz->lru_lock, flags);
+ if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED &&
+ unlikely(mem != pc->mem_cgroup)) {
+ /* MAPPED account can be done without lock_page().
+ Check race with mem_cgroup_move_account() */
+ spin_unlock_irqrestore(&mz->lru_lock, flags);
+ goto retry;
+ }
__mem_cgroup_remove_list(mz, pc);
spin_unlock_irqrestore(&mz->lru_lock, flags);

page_assign_page_cgroup(page, NULL);
unlock_page_cgroup(page);

- mem = pc->mem_cgroup;
+
res_counter_uncharge(&mem->res, PAGE_SIZE);
css_put(&mem->css);