mm: memcontrol: introduce CONFIG_MEMCG_LEGACY_KMEM

Let the user know that CONFIG_MEMCG_KMEM does not apply to the cgroup2
interface. This also makes legacy-only code sections stand out better.

[arnd@arndb.de: mm: memcontrol: only manage socket pressure for CONFIG_INET]
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Acked-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/Kconfig

@@ -964,10 +964,13 @@ config MEMCG_SWAP_ENABLED
 	  For those who want to have the feature enabled by default should
 	  select this option (if, for some reason, they need to disable it
 	  then swapaccount=0 does the trick).
+config MEMCG_LEGACY_KMEM
+       bool
 config MEMCG_KMEM
-	bool "Memory Resource Controller Kernel Memory accounting"
+	bool "Legacy Memory Resource Controller Kernel Memory accounting"
 	depends on MEMCG
 	depends on SLUB || SLAB
+	select MEMCG_LEGACY_KMEM
 	help
 	  The Kernel Memory extension for Memory Resource Controller can limit
 	  the amount of memory used by kernel objects in the system. Those are
@@ -1071,6 +1074,11 @@ config CGROUP_FREEZER
 	  Provides a way to freeze and unfreeze all tasks in a
 	  cgroup.
 
+	  This option affects the ORIGINAL cgroup interface. The cgroup2 memory
+	  controller includes important in-kernel memory consumers per default.
+
+	  If you're using cgroup2, say N.
+
 config CGROUP_HUGETLB
 	bool "HugeTLB controller"
 	depends on HUGETLB_PAGE