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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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