cgroup: increase max memory cgroup id for x86_64

On a shared many-core machine with a lot of memory and long uptimes, it is
common to hit the USHRT_MAX id limit for pinned memory cgroup states.

include/linux/memcontrol.h

@@ -73,8 +73,13 @@ struct mem_cgroup_reclaim_cookie {
 
 #ifdef CONFIG_MEMCG
 
+#if defined(__x86_64__) && defined(MEMCG_32BIT_IDS)
+#define MEM_CGROUP_ID_SHIFT	32
+#define MEM_CGROUP_ID_MAX	INT_MAX
+#else
 #define MEM_CGROUP_ID_SHIFT	16
 #define MEM_CGROUP_ID_MAX	USHRT_MAX
+#endif
 
 struct mem_cgroup_id {
 	int id;

init/Kconfig

@@ -678,6 +678,18 @@ config MEMCG_SWAP_ENABLED
 	  select this option (if, for some reason, they need to disable it
 	  then swapaccount=0 does the trick).
 
+config MEMCG_32BIT_IDS
+	bool "Use 32 bit IDs"
+	depends on MEMCG && X86_64
+	default n
+	help
+	  Extends the default limit of max 65536 memory cgroups to
+	  2147483647. This may cause an increase of bucket_order in
+	  mm/workingset.c, having TODO consequences.
+
+	  See https://lore.kernel.org/patchwork/patch/690171/ for original
+	  reasoning behind 16 bit limit.
+
 config BLK_CGROUP
 	bool "IO controller"
 	depends on BLOCK