This patch adds following functions.
- clear_page_cgroup(page, pc)
- page_cgroup_assign_new_page_group(page, pc)

Mainly for cleanup.

A manner "check page->cgroup again after lock_page_cgroup()" is
implemented in straight way.

Chnagelog v2 -> v3:
* adjusted to 2.6.23-mm1
Changelog v1 -> v2:
* added necessary comments.

Note:
- a comment in mem_cgroup_uncharge() will be removed by force-empty patch

Signed-off-by: KAMEZAWA Hiroyuki



mm/memcontrol.c | 100 ++++++++++++++++++++++++++++++++++++++++----------------
1 file changed, 73 insertions(+), 27 deletions(-)

Index: devel-2.6.23-mm1/mm/memcontrol.c
================================================== =================
--- devel-2.6.23-mm1.orig/mm/memcontrol.c
+++ devel-2.6.23-mm1/mm/memcontrol.c
@@ -162,6 +162,48 @@ static void __always_inline unlock_page_
bit_spin_unlock(PAGE_CGROUP_LOCK_BIT, &page->page_cgroup);
}

+/*
+ * Tie new page_cgroup to struct page under lock_page_cgroup()
+ * This can fail if the page has been tied to a page_cgroup.
+ * If success, returns 0.
+ */
+static inline int
+page_cgroup_assign_new_page_cgroup(struct page *page, struct page_cgroup *pc)
+{
+ int ret = 0;
+
+ lock_page_cgroup(page);
+ if (!page_get_page_cgroup(page))
+ page_assign_page_cgroup(page, pc);
+ else /* A page is tied to other pc. */
+ ret = 1;
+ unlock_page_cgroup(page);
+ return ret;
+}
+
+/*
+ * Clear page->page_cgroup member under lock_page_cgroup().
+ * If given "pc" value is different from one page->page_cgroup,
+ * page->cgroup is not cleared.
+ * Returns a value of page->page_cgroup at lock taken.
+ * A can can detect failure of clearing by following
+ * clear_page_cgroup(page, pc) == pc
+ */
+
+static inline struct page_cgroup *
+clear_page_cgroup(struct page *page, struct page_cgroup *pc)
+{
+ struct page_cgroup *ret;
+ /* lock and clear */
+ lock_page_cgroup(page);
+ ret = page_get_page_cgroup(page);
+ if (likely(ret == pc))
+ page_assign_page_cgroup(page, NULL);
+ unlock_page_cgroup(page);
+ return ret;
+}
+
+
static void __mem_cgroup_move_lists(struct page_cgroup *pc, bool active)
{
if (active)
@@ -270,7 +312,7 @@ int mem_cgroup_charge(struct page *page,
gfp_t gfp_mask)
{
struct mem_cgroup *mem;
- struct page_cgroup *pc, *race_pc;
+ struct page_cgroup *pc;
unsigned long flags;
unsigned long nr_retries = MEM_CGROUP_RECLAIM_RETRIES;

@@ -364,24 +406,20 @@ noreclaim:
goto free_pc;
}

- lock_page_cgroup(page);
- /*
- * Check if somebody else beat us to allocating the page_cgroup
- */
- race_pc = page_get_page_cgroup(page);
- if (race_pc) {
- kfree(pc);
- pc = race_pc;
- atomic_inc(&pc->ref_cnt);
- res_counter_uncharge(&mem->res, PAGE_SIZE);
- css_put(&mem->css);
- goto done;
- }
-
atomic_set(&pc->ref_cnt, 1);
pc->mem_cgroup = mem;
pc->page = page;
- page_assign_page_cgroup(page, pc);
+ if (page_cgroup_assign_new_page_cgroup(page, pc)) {
+ /*
+ * an another charge is added to this page already.
+ * we do take lock_page_cgroup(page) again and read
+ * page->cgroup, increment refcnt.... just retry is OK.
+ */
+ res_counter_uncharge(&mem->res, PAGE_SIZE);
+ css_put(&mem->css);
+ kfree(pc);
+ goto retry;
+ }

spin_lock_irqsave(&mem->lru_lock, flags);
list_add(&pc->lru, &mem->active_list);
@@ -432,17 +470,25 @@ void mem_cgroup_uncharge(struct page_cgr

if (atomic_dec_and_test(&pc->ref_cnt)) {
page = pc->page;
- lock_page_cgroup(page);
- mem = pc->mem_cgroup;
- css_put(&mem->css);
- page_assign_page_cgroup(page, NULL);
- unlock_page_cgroup(page);
- res_counter_uncharge(&mem->res, PAGE_SIZE);
-
- spin_lock_irqsave(&mem->lru_lock, flags);
- list_del_init(&pc->lru);
- spin_unlock_irqrestore(&mem->lru_lock, flags);
- kfree(pc);
+ /*
+ * get page->cgroup and clear it under lock.
+ */
+ if (clear_page_cgroup(page, pc) == pc) {
+ mem = pc->mem_cgroup;
+ css_put(&mem->css);
+ res_counter_uncharge(&mem->res, PAGE_SIZE);
+ spin_lock_irqsave(&mem->lru_lock, flags);
+ list_del_init(&pc->lru);
+ spin_unlock_irqrestore(&mem->lru_lock, flags);
+ kfree(pc);
+ } else {
+ /*
+ * Note:This will be removed when force-empty patch is
+ * applied. just show warning here.
+ */
+ printk(KERN_ERR "Race in mem_cgroup_uncharge() ?");
+ dump_stack();
+ }
}
}


-
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