mm/vmpressure: replace vmpressure_to_css() with vmpressure_to_memcg()

We can get memcg directly form vmpr instead of vmpr->memcg->css->memcg, so
add a new func helper vmpressure_to_memcg().  And no code will use
vmpressure_to_css(), so delete it.

Link: https://lkml.kernel.org/r/20210630112146.455103-1-suhui@zeku.com
Signed-off-by: Hui Su <suhui@zeku.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Chris Down <chris@chrisdown.name>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/vmpressure.h

@@ -37,7 +37,7 @@ extern void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio);
 extern void vmpressure_init(struct vmpressure *vmpr);
 extern void vmpressure_cleanup(struct vmpressure *vmpr);
 extern struct vmpressure *memcg_to_vmpressure(struct mem_cgroup *memcg);
-extern struct cgroup_subsys_state *vmpressure_to_css(struct vmpressure *vmpr);
+extern struct mem_cgroup *vmpressure_to_memcg(struct vmpressure *vmpr);
 extern int vmpressure_register_event(struct mem_cgroup *memcg,
 				     struct eventfd_ctx *eventfd,
 				     const char *args);

mm/memcontrol.c

@@ -256,9 +256,9 @@ struct vmpressure *memcg_to_vmpressure(struct mem_cgroup *memcg)
 	return &memcg->vmpressure;
 }
 
-struct cgroup_subsys_state *vmpressure_to_css(struct vmpressure *vmpr)
+struct mem_cgroup *vmpressure_to_memcg(struct vmpressure *vmpr)
 {
-	return &container_of(vmpr, struct mem_cgroup, vmpressure)->css;
+	return container_of(vmpr, struct mem_cgroup, vmpressure);
 }
 
 #ifdef CONFIG_MEMCG_KMEM

mm/vmpressure.c

@@ -74,8 +74,7 @@ static struct vmpressure *work_to_vmpressure(struct work_struct *work)
 
 static struct vmpressure *vmpressure_parent(struct vmpressure *vmpr)
 {
-	struct cgroup_subsys_state *css = vmpressure_to_css(vmpr);
-	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+	struct mem_cgroup *memcg = vmpressure_to_memcg(vmpr);
 
 	memcg = parent_mem_cgroup(memcg);
 	if (!memcg)