>> - /* We can't have an empty hierarchy */
>> +
>> + /* Can't specify "none" and some subsystems */
>> + if (opts->subsys_bits && opts->none)
>> + return -EINVAL;
>
> Is this case never happens ?
>
> BUG_ON(!opts->subsys_bits && !opts->none)
>
It can happen:
# mount -t cgroup -o none,memory
>
>> +
>> + /*
>> + * We either have to specify by name or by subsystems. (So all
>> + * empty hierarchies must have a name).
>> + */
>> if (!opts->subsys_bits && !opts->name)
>> return -EINVAL;
>>
>> @@ -1129,6 +1124,31 @@ static void init_cgroup_root(struct cgroupfs_root
>> *root)
>> init_cgroup_housekeeping(cgrp);
>> }
>>
>> +static bool init_root_id(struct cgroupfs_root *root)
>> +{
>> + int ret = 0;
>> +
>> + do {
>> + if (!ida_pre_get(&hierarchy_ida, GFP_KERNEL))
>> + return false;
>> + spin_lock(&hierarchy_id_lock);
>> + /* Try to allocate the next unused ID */
>> + ret = ida_get_new_above(&hierarchy_ida, next_hierarchy_id,
>> + &root->hierarchy_id);
>
> Why new id should be greater than old one ?
> Just for avoiding reuse-id-too-soon ?
>
Otherwise we can't identify each hierachy by it's id.
>> + if (ret == -ENOSPC)
>> + /* Try again starting from 0 */
>> + ret = ida_get_new(&hierarchy_ida, &root->hierarchy_id);
>> + if (!ret) {
>> + next_hierarchy_id = root->hierarchy_id + 1;
>> + } else if (ret != -EAGAIN) {
>> + /* Can only get here if the 31-bit IDR is full ... */
>> + BUG_ON(ret);
>> + }
>> + spin_unlock(&hierarchy_id_lock);
>> + } while (ret);
>> + return true;
>> +}
>> +
...
>> @@ -3910,8 +3950,8 @@ static int current_css_set_cg_links_read(struct cgroup
>> *cont,
>> name = c->dentry->d_name.name;
>> else
>> name = "?";
>> - seq_printf(seq, "Root %lu group %s\n",
>> - c->root->subsys_bits, name);
>> + seq_printf(seq, "Root %d group %s\n",
>> + c->root->hierarchy_id, name);
>> }
>
> I'm not sure but this "id" is worth to be printed ?
>
It's a debug file, so I think at least it's not bad to print the id.
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