[PATCH v2] mm, memcg: fix build error around the usage of kmem_caches

[Yafang Shao <laoar.shao@gmail.com>]

When I manually set default n to MEMCG_KMEM in init/Kconfig, bellow error
occurs,

mm/slab_common.c: In function 'memcg_slab_start':
mm/slab_common.c:1530:30: error: 'struct mem_cgroup' has no member named
'kmem_caches'
  return seq_list_start(&memcg->kmem_caches, *pos);
                              ^
mm/slab_common.c: In function 'memcg_slab_next':
mm/slab_common.c:1537:32: error: 'struct mem_cgroup' has no member named
'kmem_caches'
  return seq_list_next(p, &memcg->kmem_caches, pos);
                                ^
mm/slab_common.c: In function 'memcg_slab_show':
mm/slab_common.c:1551:16: error: 'struct mem_cgroup' has no member named
'kmem_caches'
  if (p == memcg->kmem_caches.next)
                ^
  CC      arch/x86/xen/smp.o
mm/slab_common.c: In function 'memcg_slab_start':
mm/slab_common.c:1531:1: warning: control reaches end of non-void function
[-Wreturn-type]
 }
 ^
mm/slab_common.c: In function 'memcg_slab_next':
mm/slab_common.c:1538:1: warning: control reaches end of non-void function
[-Wreturn-type]
 }
 ^

That's because kmem_caches is defined only when CONFIG_MEMCG_KMEM is set,
while memcg_slab_start() will use it no matter CONFIG_MEMCG_KMEM is defined
or not.

By the way, the reason I mannuly undefined CONFIG_MEMCG_KMEM is to verify
whether my some other code change is still stable when CONFIG_MEMCG_KMEM is
not set. Unfortunately, the existing code has been already unstable since
v4.11.

Fixes: bc2791f857e1 ("slab: link memcg kmem_caches on their associated memory cgroup")
Cc: Tejun Heo <tj@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 mm/memcontrol.c  | 3 ++-
 mm/slab_common.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6f6dc8712e39..a94b1a737bc0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4723,7 +4723,8 @@ static struct cftype mem_cgroup_legacy_files[] = {
 		.write = mem_cgroup_reset,
 		.read_u64 = mem_cgroup_read_u64,
 	},
-#if defined(CONFIG_SLAB) || defined(CONFIG_SLUB_DEBUG)
+#if defined(CONFIG_MEMCG_KMEM) && \
+	(defined(CONFIG_SLAB) || defined(CONFIG_SLUB_DEBUG))
 	{
 		.name = "kmem.slabinfo",
 		.seq_start = memcg_slab_start,
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 1907cb2903c7..5282f881d2f5 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1521,7 +1521,7 @@ void dump_unreclaimable_slab(void)
 	mutex_unlock(&slab_mutex);
 }
 
-#if defined(CONFIG_MEMCG)
+#if defined(CONFIG_MEMCG_KMEM)
 void *memcg_slab_start(struct seq_file *m, loff_t *pos)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
-- 
2.14.1