cgroup/cpuset-v1: Add missing support for cpuset_v2_mode

Android has mounted the v1 cpuset controller using filesystem type
"cpuset" (not "cgroup") since 2015 [1], and depends on the resulting
behavior where the controller name is not added as a prefix for cgroupfs
files. [2]

Later, a problem was discovered where cpu hotplug onlining did not
affect the cpuset/cpus files, which Android carried an out-of-tree patch
to address for a while. An attempt was made to upstream this patch, but
the recommendation was to use the "cpuset_v2_mode" mount option
instead. [3]

An effort was made to do so, but this fails with "cgroup: Unknown
parameter 'cpuset_v2_mode'" because commit e1cba4b ("cgroup: Add
mount flag to enable cpuset to use v2 behavior in v1 cgroup") did not
update the special cased cpuset_mount(), and only the cgroup (v1)
filesystem type was updated.

Add parameter parsing to the cpuset filesystem type so that
cpuset_v2_mode works like the cgroup filesystem type:

$ mkdir /dev/cpuset
$ mount -t cpuset -ocpuset_v2_mode none /dev/cpuset
$ mount|grep cpuset
none on /dev/cpuset type cgroup (rw,relatime,cpuset,noprefix,cpuset_v2_mode,release_agent=/sbin/cpuset_release_agent)

[1] https://cs.android.com/android/_/android/platform/system/core/+/b769c8d24fd7be96f8968aa4c80b669525b930d3
[2] https://cs.android.com/android/platform/superproject/main/+/main:system/core/libprocessgroup/setup/cgroup_map_write.cpp;drc=2dac5d89a0f024a2d0cc46a80ba4ee13472f1681;l=192
[3] https://lore.kernel.org/lkml/f795f8be-a184-408a-0b5a-553d26061385@redhat.com/T/

Fixes: e1cba4b ("cgroup: Add mount flag to enable cpuset to use v2 behavior in v1 cgroup")
Signed-off-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Waiman Long <longman@redhat.com>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -2353,9 +2353,37 @@ static struct file_system_type cgroup2_fs_type = {
 };
 
 #ifdef CONFIG_CPUSETS_V1
+enum cpuset_param {
+	Opt_cpuset_v2_mode,
+};
+
+static const struct fs_parameter_spec cpuset_fs_parameters[] = {
+	fsparam_flag  ("cpuset_v2_mode", Opt_cpuset_v2_mode),
+	{}
+};
+
+static int cpuset_parse_param(struct fs_context *fc, struct fs_parameter *param)
+{
+	struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
+	struct fs_parse_result result;
+	int opt;
+
+	opt = fs_parse(fc, cpuset_fs_parameters, param, &result);
+	if (opt < 0)
+		return opt;
+
+	switch (opt) {
+	case Opt_cpuset_v2_mode:
+		ctx->flags |= CGRP_ROOT_CPUSET_V2_MODE;
+		return 0;
+	}
+	return -EINVAL;
+}
+
 static const struct fs_context_operations cpuset_fs_context_ops = {
 	.get_tree	= cgroup1_get_tree,
 	.free		= cgroup_fs_context_free,
+	.parse_param	= cpuset_parse_param,
 };
 
 /*
@@ -2392,6 +2420,7 @@ static int cpuset_init_fs_context(struct fs_context *fc)
 static struct file_system_type cpuset_fs_type = {
 	.name			= "cpuset",
 	.init_fs_context	= cpuset_init_fs_context,
+	.parameters		= cpuset_fs_parameters,
 	.fs_flags		= FS_USERNS_MOUNT,
 };
 #endif