On Tue, 28 Feb 2012 14:45:07 -0800
Andrew Morton [off-list ref] wrote:

On Tue, 28 Feb 2012 22:00:26 +0800
Fengguang Wu [off-list ref] wrote:

quoted

From: Greg Thelen <redacted>

Added memcg dirty page accounting support routines.  These routines are
used by later changes to provide memcg aware writeback and dirty page
limiting.  A mem_cgroup_dirty_info() tracepoint is is also included to
allow for easier understanding of memcg writeback operation.

...

+/*
+ * Return the number of additional pages that the @memcg cgroup could allocate.
+ * If use_hierarchy is set, then this involves checking parent mem cgroups to
+ * find the cgroup with the smallest free space.
+ */

Comment needs revisting - use_hierarchy does not exist.

quoted

+static unsigned long
+mem_cgroup_hierarchical_free_pages(struct mem_cgroup *memcg)
+{
+	u64 free;
+	unsigned long min_free;
+
+	min_free = global_page_state(NR_FREE_PAGES);
+
+	while (memcg) {
+		free = mem_cgroup_margin(memcg);
+		min_free = min_t(u64, min_free, free);
+		memcg = parent_mem_cgroup(memcg);
+	}
+
+	return min_free;
+}
+
+/*
+ * mem_cgroup_page_stat() - get memory cgroup file cache statistics
+ * @memcg:     memory cgroup to query
+ * @item:      memory statistic item exported to the kernel
+ *
+ * Return the accounted statistic value.
+ */
+unsigned long mem_cgroup_page_stat(struct mem_cgroup *memcg,
+				   enum mem_cgroup_page_stat_item item)
+{
+	struct mem_cgroup *iter;
+	s64 value;
+
+	/*
+	 * If we're looking for dirtyable pages we need to evaluate free pages
+	 * depending on the limit and usage of the parents first of all.
+	 */
+	if (item == MEMCG_NR_DIRTYABLE_PAGES)
+		value = mem_cgroup_hierarchical_free_pages(memcg);
+	else
+		value = 0;
+
+	/*
+	 * Recursively evaluate page statistics against all cgroup under
+	 * hierarchy tree
+	 */
+	for_each_mem_cgroup_tree(iter, memcg)
+		value += mem_cgroup_local_page_stat(iter, item);

What's the locking rule for for_each_mem_cgroup_tree()?  It's unobvious
from the code and isn't documented?

Because for_each_mem_cgroup_tree() uses rcu_read_lock() and referernce counting
internally, it's not required to take any lock in callers.
One rule is the caller shoud call mem_cgroup_iter_break() if he want to break
the loop.

Thanks,
-Kame






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