[PATCH] vmscan: fix memcg_kmem build failure

[Arnd Bergmann <arnd@arndb.de>]

When CONFIG_MEMCG_KMEM is disabled, we get a build failure
for calling a nonexisting memcg_expand_shrinker_maps():

mm/vmscan.c:220:7: error: implicit declaration of function 'memcg_expand_shrinker_maps' [-Werror,-Wimplicit-function-declaration]
                if (memcg_expand_shrinker_maps(id)) {
                    ^
mm/vmscan.c:220:7: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
mm/vmscan.c:608:56: error: no member named 'shrinker_map' in 'struct mem_cgroup_per_node'
        map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map,
                                        ~~~~~~~~~~~~~~~~~~~~  ^
include/linux/rcupdate.h:498:31: note: expanded from macro 'rcu_dereference_protected'
        __rcu_dereference_protected((p), (c), __rcu)
                                     ^
include/linux/rcupdate.h:321:12: note: expanded from macro '__rcu_dereference_protected'
        ((typeof(*p) __force __kernel *)(p)); \
                  ^
mm/vmscan.c:608:6: error: assigning to 'struct memcg_shrinker_map *' from incompatible type 'void'
        map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map,

and another issue trying to access invalid struct fields:

mm/vmscan.c:608:56: error: no member named 'shrinker_map' in 'struct mem_cgroup_per_node'
        map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map,
                                        ~~~~~~~~~~~~~~~~~~~~  ^
include/linux/rcupdate.h:498:31: note: expanded from macro 'rcu_dereference_protected'
        __rcu_dereference_protected((p), (c), __rcu)
                                     ^
include/linux/rcupdate.h:321:12: note: expanded from macro '__rcu_dereference_protected'
        ((typeof(*p) __force __kernel *)(p)); \
                  ^
mm/vmscan.c:608:6: error: assigning to 'struct memcg_shrinker_map *' from incompatible type 'void'
        map = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_map,

Add a dummy definition for memcg_expand_shrinker_maps() that always fails,
and hide the obviously nonworking shrink_slab_memcg() function.

Fixes: 8236f517d69e ("mm: shrinker: make shrinker not depend on memcg kmem")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
No idea what the intended behavior is supposed to be for this case.
Rather than failing, should we actually provide that function?
Or maybe a more elaborate change is needed?
---
 include/linux/memcontrol.h | 5 +++++
 mm/vmscan.c                | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 5901a90f58eb..6b15e2066fc7 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1407,6 +1407,11 @@ static inline void memcg_put_cache_ids(void)
 {
 }
 
+static inline int memcg_expand_shrinker_maps(int new_id)
+{
+	return -ENOMEM;
+}
+
 static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
 					  int nid, int shrinker_id) { }
 #endif /* CONFIG_MEMCG_KMEM */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a0301edd8d03..323a9c50c0fe 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -591,7 +591,7 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 	return freed;
 }
 
-#ifdef CONFIG_MEMCG
+#ifdef CONFIG_MEMCG_KMEM
 static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
 			struct mem_cgroup *memcg, int priority)
 {
-- 
2.20.0