(2012/03/22 22:38), Michal Hocko wrote:

On Mon 19-03-12 17:03:42, KAMEZAWA Hiroyuki wrote:
[...]

quoted

@@ -1237,8 +1237,6 @@ mem_cgroup_get_reclaim_stat_from_page(struct page *page)
 	pc = lookup_page_cgroup(page);
 	if (!PageCgroupUsed(pc))
 		return NULL;
-	/* Ensure pc's mem_cgroup is visible after reading PCG_USED. */
-	smp_rmb();
 	mz = page_cgroup_zoneinfo(pc_to_mem_cgroup(pc), page);
 	return &mz->reclaim_stat;
 }

@@ -2491,16 +2489,7 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 		}
 	}
 
-	pc_set_mem_cgroup(pc, memcg);
-	/*
-	 * We access a page_cgroup asynchronously without lock_page_cgroup().
-	 * Especially when a page_cgroup is taken from a page, pc's mem_cgroup
-	 * is accessed after testing USED bit. To make pc's mem_cgroup visible
-	 * before USED bit, we need memory barrier here.
-	 * See mem_cgroup_add_lru_list(), etc.
- 	 */
-	smp_wmb();
-	SetPageCgroupUsed(pc);
+	pc_set_mem_cgroup(pc, memcg, BIT(PCG_USED) | BIT(PCG_LOCK));

This is not nice. Maybe we need two variants (pc_set_mem_cgroup[_flags])?

Sure. I'll add that.

quoted

 	if (lrucare) {
 		if (was_on_lru) {

@@ -2529,7 +2518,6 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 
-#define PCGF_NOCOPY_AT_SPLIT ((1 << PCG_LOCK) | (1 << PCG_MIGRATION))
 /*
  * Because tail pages are not marked as "used", set it. We're under
  * zone->lru_lock, 'splitting on pmd' and compound_lock.

@@ -2547,9 +2535,7 @@ void mem_cgroup_split_huge_fixup(struct page *head)
 		return;
 	for (i = 1; i < HPAGE_PMD_NR; i++) {
 		pc = head_pc + i;
-		pc_set_mem_cgroup(pc, memcg);
-		smp_wmb();/* see __commit_charge() */
-		pc->flags = head_pc->flags & ~PCGF_NOCOPY_AT_SPLIT;
+		pc_set_mem_cgroup(pc, memcg, BIT(PCG_USED));

Maybe it would be cleaner to remove PCGF_NOCOPY_AT_SPLIT in a separate patch with 
VM_BUG_ON(!head_pc->flags & BIT(PCG_USED))?

Hm, ok. I'll divide this patch.

quoted

 	}
 }
 #endif /* CONFIG_TRANSPARENT_HUGEPAGE */

@@ -2616,7 +2602,7 @@ 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(pc, to, BIT(PCG_USED) | BIT(PCG_LOCK));

Same here.

pc_set_mem_cgroup_flags() ?

Thanks,
-Kame

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