[PATCH v8 2/6] cgroup,cgroup/dmem: Add (dmem_)cgroup_common_ancestor helper

This helps to find a common subtree of two resources, which is important
when determining whether it's helpful to evict one resource in favor of
another.

To facilitate this, add a common helper to find the ancestor of two
cgroups using each cgroup's ancestor array.

Signed-off-by: Natalie Vock <[email protected]>
=2D--
 include/linux/cgroup.h      | 21 +++++++++++++++++++++
 include/linux/cgroup_dmem.h |  9 +++++++++
 kernel/cgroup/dmem.c        | 41 ++++++++++++++++++++++++++++++++++++++++=
+
 3 files changed, 71 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f2aa46a4f871e..83a17ded1c516 100644
=2D-- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -623,6 +623,27 @@ static inline struct cgroup *cgroup_ancestor(struct c=
group *cgrp,
 	return cgrp->ancestors[ancestor_level];
 }
=20
+/**
+ * cgroup_common_ancestor - find common ancestor of two cgroups
+ * @a: first cgroup to find common ancestor of
+ * @b: second cgroup to find common ancestor of
+ *
+ * Find the first cgroup that is an ancestor of both @a and @b, if it exi=
sts
+ * and return a pointer to it. If such a cgroup doesn't exist, return NUL=
L.
+ *
+ * This function is safe to call as long as both @a and @b are accessible=
.
+ */
+static inline struct cgroup *cgroup_common_ancestor(struct cgroup *a,
+						    struct cgroup *b)
+{
+	int level;
+
+	for (level =3D min(a->level, b->level); level >=3D 0; level--)
+		if (a->ancestors[level] =3D=3D b->ancestors[level])
+			return a->ancestors[level];
+	return NULL;
+}
+
 /**
  * task_under_cgroup_hierarchy - test task's membership of cgroup ancestr=
y
  * @task: the task to be tested
diff --git a/include/linux/cgroup_dmem.h b/include/linux/cgroup_dmem.h
index 1a88cd0c9eb00..9d72457c4cb9d 100644
=2D-- a/include/linux/cgroup_dmem.h
+++ b/include/linux/cgroup_dmem.h
@@ -28,6 +28,8 @@ bool dmem_cgroup_below_min(struct dmem_cgroup_pool_state=
 *root,
 			   struct dmem_cgroup_pool_state *test);
 bool dmem_cgroup_below_low(struct dmem_cgroup_pool_state *root,
 			   struct dmem_cgroup_pool_state *test);
+struct dmem_cgroup_pool_state *dmem_cgroup_get_common_ancestor(struct dme=
m_cgroup_pool_state *a,
+							       struct dmem_cgroup_pool_state *b);
=20
 void dmem_cgroup_pool_state_put(struct dmem_cgroup_pool_state *pool);
 #else
@@ -75,6 +77,13 @@ static inline bool dmem_cgroup_below_low(struct dmem_cg=
roup_pool_state *root,
 	return false;
 }
=20
+static inline
+struct dmem_cgroup_pool_state *dmem_cgroup_get_common_ancestor(struct dme=
m_cgroup_pool_state *a,
+							       struct dmem_cgroup_pool_state *b)
+{
+	return NULL;
+}
+
 static inline void dmem_cgroup_pool_state_put(struct dmem_cgroup_pool_sta=
te *pool)
 { }
=20
diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
index 9df3b33c65046..a587611ca2235 100644
=2D-- a/kernel/cgroup/dmem.c
+++ b/kernel/cgroup/dmem.c
@@ -762,6 +762,47 @@ bool dmem_cgroup_below_low(struct dmem_cgroup_pool_st=
ate *root,
 }
 EXPORT_SYMBOL_GPL(dmem_cgroup_below_low);
=20
+/**
+ * dmem_cgroup_get_common_ancestor(): Find the first common ancestor of t=
wo pools.
+ * @a: First pool to find the common ancestor of.
+ * @b: First pool to find the common ancestor of.
+ *
+ * Return: The first pool that is a parent of both @a and @b, or NULL if =
either @a or @b are NULL,
+ * or if such a pool does not exist. A reference to the returned pool is =
grabbed and must be
+ * released by the caller when it is done using the pool.
+ */
+struct dmem_cgroup_pool_state *dmem_cgroup_get_common_ancestor(struct dme=
m_cgroup_pool_state *a,
+							       struct dmem_cgroup_pool_state *b)
+{
+	struct cgroup *ancestor_cgroup;
+	struct cgroup_subsys_state *ancestor_css;
+	struct dmemcg_state *ancestor_dmemcs =3D NULL;
+	struct dmem_cgroup_pool_state *pool =3D NULL;
+
+	if (!a || !b)
+		return NULL;
+
+	ancestor_cgroup =3D cgroup_common_ancestor(a->cs->css.cgroup, b->cs->css=
.cgroup);
+	if (!ancestor_cgroup)
+		return NULL;
+
+	rcu_read_lock();
+	ancestor_css =3D cgroup_e_css(ancestor_cgroup, &dmem_cgrp_subsys);
+	if (css_tryget(ancestor_css))
+		ancestor_dmemcs =3D css_to_dmemcs(ancestor_css);
+	rcu_read_unlock();
+
+	if (ancestor_dmemcs) {
+		pool =3D get_cg_pool_unlocked(css_to_dmemcs(ancestor_css),
+					    a->region);
+		if (IS_ERR(pool))
+			pool =3D NULL;
+		css_put(ancestor_css);
+	}
+	return pool;
+}
+EXPORT_SYMBOL_GPL(dmem_cgroup_get_common_ancestor);
+
 static int dmem_cgroup_region_capacity_show(struct seq_file *sf, void *v)
 {
 	struct dmem_cgroup_region *region;

=2D-=20
2.55.0