Re: [BUGFIX][PATCH] memcg: get/put parents at create/free

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

This is a fix for memcg-get-put-parents-at-create-free.patch.

===
From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

Andrew suggested that it's strange to add a little helper function for get(),
while put() is open-code.

This patch also adds a few comments.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 mm/memcontrol.c |   20 ++++++++++----------
 1 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 45e1b51..92790e4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -203,7 +203,6 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
 static void mem_cgroup_get(struct mem_cgroup *mem);
 static void mem_cgroup_put(struct mem_cgroup *mem);
 static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
-static void mem_cgroup_get_parent(struct mem_cgroup *mem);
 
 static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
 					 struct page_cgroup *pc,
@@ -2195,6 +2194,9 @@ static void mem_cgroup_put(struct mem_cgroup *mem)
 	}
 }
 
+/*
+ * Returns the parent mem_cgroup in memcgroup hierarchy with hierarchy enabled.
+ */
 static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem)
 {
 	if (!mem->res.parent)
@@ -2202,14 +2204,6 @@ static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem)
 	return mem_cgroup_from_res_counter(mem->res.parent, res);
 }
 
-static void mem_cgroup_get_parent(struct mem_cgroup *mem)
-{
-	struct mem_cgroup *parent = parent_mem_cgroup(mem);
-
-	if (parent)
-		mem_cgroup_get(parent);
-}
-
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 static void __init enable_swap_cgroup(void)
 {
@@ -2247,6 +2241,13 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 	if (parent && parent->use_hierarchy) {
 		res_counter_init(&mem->res, &parent->res);
 		res_counter_init(&mem->memsw, &parent->memsw);
+		/*
+		 * We increment refcnt of the parent to ensure that we can
+		 * safely access it on res_counter_charge/uncharge.
+		 * This refcnt will be decremented when freeing this
+		 * mem_cgroup(see mem_cgroup_put).
+		 */
+		mem_cgroup_get(parent);
 	} else {
 		res_counter_init(&mem->res, NULL);
 		res_counter_init(&mem->memsw, NULL);
@@ -2257,7 +2258,6 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 	if (parent)
 		mem->swappiness = get_swappiness(parent);
 	atomic_set(&mem->refcnt, 1);
-	mem_cgroup_get_parent(mem);
 	return &mem->css;
 free_out:
 	__mem_cgroup_free(mem);

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