Andrew Morton [off-list ref] writes:

On Wed, 18 Jul 2012 11:04:09 +0530
"Aneesh Kumar K.V" [off-list ref] wrote:

quoted

From: "Aneesh Kumar K.V" <redacted>

Since we cannot fail in hugetlb_cgroup_move_parent, we don't really
need to check whether cgroup have any change left after that. Also skip
those hstates for which we don't have any charge in this cgroup.

...

+	for_each_hstate(h) {
+		/*
+		 * if we don't have any charge, skip this hstate
+		 */
+		idx = hstate_index(h);
+		if (res_counter_read_u64(&h_cg->hugepage[idx], RES_USAGE) == 0)
+			continue;
+		spin_lock(&hugetlb_lock);
+		list_for_each_entry(page, &h->hugepage_activelist, lru)
+			hugetlb_cgroup_move_parent(idx, cgroup, page);
+		spin_unlock(&hugetlb_lock);
+		VM_BUG_ON(res_counter_read_u64(&h_cg->hugepage[idx], RES_USAGE));
+	}
 out:
 	return ret;
 }

This looks fishy.

We test RES_USAGE before taking hugetlb_lock.  What prevents some other
thread from increasing RES_USAGE after that test?

After walking the list we test RES_USAGE after dropping hugetlb_lock. 
What prevents another thread from incrementing RES_USAGE before that
test, triggering the BUG?

IIUC core cgroup will prevent a new task getting added to the cgroup
when we are in pre_destroy. Since we already check that the cgroup doesn't
have any task, the RES_USAGE cannot increase in pre_destroy.

-aneesh

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