Revert "memcg: drop kmem.limit_in_bytes"

This reverts commit 21ef9e1 which is
commit 86327e8 upstream.

It breaks existing runc systems, as the tool always thinks the file
should be present.

Reported-by: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Link: https://lore.kernel.org/r/20230920081101.GA12096@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/cgroup-v1/memory.rst

@@ -91,6 +91,8 @@ Brief summary of control files.
  memory.oom_control		     set/show oom controls.
  memory.numa_stat		     show the number of memory usage per numa
 				     node
+ memory.kmem.limit_in_bytes          This knob is deprecated and writing to
+                                     it will return -ENOTSUPP.
  memory.kmem.usage_in_bytes          show current kernel memory allocation
  memory.kmem.failcnt                 show the number of kernel memory usage
 				     hits limits

mm/memcontrol.c

@@ -3841,6 +3841,10 @@ static ssize_t mem_cgroup_write(struct kernfs_open_file *of,
 		case _MEMSWAP:
 			ret = mem_cgroup_resize_max(memcg, nr_pages, true);
 			break;
+		case _KMEM:
+			/* kmem.limit_in_bytes is deprecated. */
+			ret = -EOPNOTSUPP;
+			break;
 		case _TCP:
 			ret = memcg_update_tcp_max(memcg, nr_pages);
 			break;
@@ -5051,6 +5055,12 @@ static struct cftype mem_cgroup_legacy_files[] = {
 		.seq_show = memcg_numa_stat_show,
 	},
 #endif
+	{
+		.name = "kmem.limit_in_bytes",
+		.private = MEMFILE_PRIVATE(_KMEM, RES_LIMIT),
+		.write = mem_cgroup_write,
+		.read_u64 = mem_cgroup_read_u64,
+	},
 	{
 		.name = "kmem.usage_in_bytes",
 		.private = MEMFILE_PRIVATE(_KMEM, RES_USAGE),