memcg: drop kmem.limit_in_bytes

kmem.limit_in_bytes (v1 way to limit kernel memory usage) has been
deprecated since 58056f7 ("memcg, kmem: further deprecate
kmem.limit_in_bytes") merged in 5.16.  We haven't heard about any serious
users since then but it seems that the mere presence of the file is
causing more harm thatn good.  We (SUSE) have had several bug reports from
customers where Docker based containers started to fail because a write to
kmem.limit_in_bytes has failed.

This was unexpected because runc code only expects ENOENT (kmem disabled)
or EBUSY (tasks already running within cgroup).  So a new error code was
unexpected and the whole container startup failed.  This has been later
addressed by
opencontainers/runc@52390d6
so current Docker runtimes do not suffer from the problem anymore.  There
are still older version of Docker in use and likely hard to get rid of
completely.

Address this by wiping out the file completely and effectively get back to
pre 4.5 era and CONFIG_MEMCG_KMEM=n configuration.

I would recommend backporting to stable trees which have picked up
58056f7 ("memcg, kmem: further deprecate kmem.limit_in_bytes").

[[email protected]: restore _KMEM switch case]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Michal Hocko

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

@@ -92,8 +92,6 @@ 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

@@ -3871,10 +3871,6 @@ 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;
@@ -5085,12 +5081,6 @@ 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),