[RFC][PATCH 3/4] memcg: fix for mem_cgroup_hierarchical_reclaim

From: Daisuke Nishimura
Date: Thu Jan 08 2009 - 05:18:39 EST


If root_mem has no children, last_scaned_child is set to root_mem itself.
But after some children added to root_mem, mem_cgroup_get_next_node can
mem_cgroup_put the root_mem although root_mem has not been mem_cgroup_get.

This patch fixes this behavior by:
- Set last_scanned_child to NULL if root_mem has no children or DFS search
has returned to root_mem itself(root_mem is not a "child" of root_mem).
Make mem_cgroup_get_first_node return root_mem in this case.
There are no mem_cgroup_get/put for root_mem.
- Rename mem_cgroup_get_next_node to __mem_cgroup_get_next_node, and
mem_cgroup_get_first_node to mem_cgroup_get_next_node.
Make mem_cgroup_hierarchical_reclaim call only new mem_cgroup_get_next_node.


Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 37 +++++++++++++++++++++----------------
1 files changed, 21 insertions(+), 16 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 288e22c..dc38a0e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -622,7 +622,7 @@ unsigned long mem_cgroup_isolate_pages(unsigned long nr_to_scan,
* called with hierarchy_mutex held
*/
static struct mem_cgroup *
-mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
+__mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
{
struct cgroup *cgroup, *curr_cgroup, *root_cgroup;

@@ -644,8 +644,8 @@ mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
visit_parent:
if (curr_cgroup == root_cgroup) {
mem_cgroup_put(curr);
- curr = root_mem;
- mem_cgroup_get(curr);
+ /* caller handles NULL case */
+ curr = NULL;
goto done;
}

@@ -668,7 +668,6 @@ visit_parent:
goto visit_parent;

done:
- root_mem->last_scanned_child = curr;
return curr;
}

@@ -678,20 +677,29 @@ done:
* that to reclaim free pages from.
*/
static struct mem_cgroup *
-mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
+mem_cgroup_get_next_node(struct mem_cgroup *root_mem)
{
struct cgroup *cgroup;
struct mem_cgroup *ret;
bool obsolete;

- obsolete = mem_cgroup_is_obsolete(root_mem->last_scanned_child);
-
/*
* Scan all children under the mem_cgroup mem
*/
mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
+
+ obsolete = mem_cgroup_is_obsolete(root_mem->last_scanned_child);
+
if (list_empty(&root_mem->css.cgroup->children)) {
- ret = root_mem;
+ /*
+ * root_mem might have children before and last_scanned_child
+ * may point to one of them.
+ */
+ if (root_mem->last_scanned_child) {
+ VM_BUG_ON(!obsolete);
+ mem_cgroup_put(root_mem->last_scanned_child);
+ }
+ ret = NULL;
goto done;
}

@@ -705,13 +713,13 @@ mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
ret = mem_cgroup_from_cont(cgroup);
mem_cgroup_get(ret);
} else
- ret = mem_cgroup_get_next_node(root_mem->last_scanned_child,
+ ret = __mem_cgroup_get_next_node(root_mem->last_scanned_child,
root_mem);

done:
root_mem->last_scanned_child = ret;
mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
- return ret;
+ return (ret) ? ret : root_mem;
}

static bool mem_cgroup_check_under_limit(struct mem_cgroup *mem)
@@ -769,21 +777,18 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
if (!root_mem->use_hierarchy)
return ret;

- next_mem = mem_cgroup_get_first_node(root_mem);
+ next_mem = mem_cgroup_get_next_node(root_mem);

while (next_mem != root_mem) {
if (mem_cgroup_is_obsolete(next_mem)) {
- mem_cgroup_put(next_mem);
- next_mem = mem_cgroup_get_first_node(root_mem);
+ next_mem = mem_cgroup_get_next_node(root_mem);
continue;
}
ret = try_to_free_mem_cgroup_pages(next_mem, gfp_mask, noswap,
get_swappiness(next_mem));
if (mem_cgroup_check_under_limit(root_mem))
return 0;
- mutex_lock(&mem_cgroup_subsys.hierarchy_mutex);
- next_mem = mem_cgroup_get_next_node(next_mem, root_mem);
- mutex_unlock(&mem_cgroup_subsys.hierarchy_mutex);
+ next_mem = mem_cgroup_get_next_node(root_mem);
}
return ret;
}
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