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SubjectRe: [RFC][PATCH 3/4] memcg: fix for mem_cgroup_hierarchical_reclaim
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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....
Thanks,
-Kame

> ---
> 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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