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SubjectRe: [RFC][PATCH 3/4] memcg: fix for mem_cgroup_hierarchical_reclaim
On Thu, 8 Jan 2009 20:08:01 +0900 (JST)
"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> Daisuke Nishimura said:
> > 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@mxp.nes.nec.co.jp>
>
> Hmm, seems necessary fix. Then, it's better to rebase my patch on to this.
>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> Maybe simpler one can be written but my patch remove all this out later....
>
How about this ? (just an exmaple and not tested, sorry)



---
mm/memcontrol.c | 52 ++++++++++++++++++++++------------------------------
1 file changed, 22 insertions(+), 30 deletions(-)

Index: mmotm-2.6.28-Jan7/mm/memcontrol.c
===================================================================
--- mmotm-2.6.28-Jan7.orig/mm/memcontrol.c
+++ mmotm-2.6.28-Jan7/mm/memcontrol.c
@@ -621,6 +621,7 @@ static struct mem_cgroup *
mem_cgroup_get_next_node(struct mem_cgroup *curr, struct mem_cgroup *root_mem)
{
struct cgroup *cgroup, *curr_cgroup, *root_cgroup;
+ struct mem_cgroup *orig = root_mem->last_scanned_child;

curr_cgroup = curr->css.cgroup;
root_cgroup = root_mem->css.cgroup;
@@ -629,19 +630,15 @@ mem_cgroup_get_next_node(struct mem_cgro
/*
* Walk down to children
*/
- mem_cgroup_put(curr);
cgroup = list_entry(curr_cgroup->children.next,
struct cgroup, sibling);
curr = mem_cgroup_from_cont(cgroup);
- mem_cgroup_get(curr);
goto done;
}

visit_parent:
if (curr_cgroup == root_cgroup) {
- mem_cgroup_put(curr);
curr = root_mem;
- mem_cgroup_get(curr);
goto done;
}

@@ -649,11 +646,9 @@ visit_parent:
* Goto next sibling
*/
if (curr_cgroup->sibling.next != &curr_cgroup->parent->children) {
- mem_cgroup_put(curr);
cgroup = list_entry(curr_cgroup->sibling.next, struct cgroup,
sibling);
curr = mem_cgroup_from_cont(cgroup);
- mem_cgroup_get(curr);
goto done;
}

@@ -664,7 +659,10 @@ visit_parent:
goto visit_parent;

done:
+ if (orig)
+ mem_cgroup_put(orig);
root_mem->last_scanned_child = curr;
+ mem_cgroup_get(curr);
return curr;
}

@@ -677,35 +675,25 @@ static struct mem_cgroup *
mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
{
struct cgroup *cgroup;
- struct mem_cgroup *ret;
- bool obsolete;
+ struct mem_cgroup *ret, *orig;

- 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);
- if (list_empty(&root_mem->css.cgroup->children)) {
- ret = root_mem;
- goto done;
- }
-
- if (!root_mem->last_scanned_child || obsolete) {
-
- if (obsolete && root_mem->last_scanned_child)
- mem_cgroup_put(root_mem->last_scanned_child);
+ orig = root_mem->last_scanned_child;

- cgroup = list_first_entry(&root_mem->css.cgroup->children,
- struct cgroup, sibling);
- ret = mem_cgroup_from_cont(cgroup);
+ if (!orig) {
+ if (list_empty(&root_mem->css.cgroup->children)) {
+ ret = root_mem;
+ } else {
+ cgroup =
+ list_first_entry(&root_mem->css.cgroup->children,
+ struct cgroup, sibling);
+ ret = mem_cgroup_from_cont(cgroup);
+ }
+ root_mem->last_scanned_child = ret;
mem_cgroup_get(ret);
- } else
+ } else /* get_next_node will manage refcnt */
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;
}
@@ -2232,7 +2220,11 @@ static void mem_cgroup_pre_destroy(struc
static void mem_cgroup_destroy(struct cgroup_subsys *ss,
struct cgroup *cont)
{
- mem_cgroup_put(mem_cgroup_from_cont(cont));
+ struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
+
+ if (mem->last_scanned_child == mem)
+ mem_cgroup_put(mem);
+ mem_cgroup_put(mem);
}

static int mem_cgroup_populate(struct cgroup_subsys *ss,


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