Re: [PATCH v2] cgroup/cpuset-v1: Add missing support for cpuset_v2_mode

[Waiman Long <longman@redhat.com>]

On 4/15/25 7:53 PM, T.J. Mercier wrote:
> 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 e1cba4b85daa ("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: e1cba4b85daa ("cgroup: Add mount flag to enable cpuset to use v2 behavior in v1 cgroup")
> Signed-off-by: T.J. Mercier <tjmercier@google.com>
> ---
>   kernel/cgroup/cgroup.c | 29 +++++++++++++++++++++++++++++
>   1 file changed, 29 insertions(+)
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 27f08aa17b56..cf30ff2e7d60 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/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,
> +};
> +
> +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

LGTM

Acked-by: Waiman Long <longman@redhat.com>