Re: [PATCH v8 02/21] x86/resctrl: Remove the limit on the number of CLOSID

[Reinette Chatre]

Hi James,

On 4/24/25 2:12 AM, James Morse wrote:
> Hello!
> 
> On 11/04/2025 17:42, James Morse wrote:
>> From: Amit Singh Tomar <amitsinght@xxxxxxxxxxx>
>>
>> Resctrl allocates and finds free CLOSID values using the bits of a u32.
>> This restricts the number of control groups that can be created by
>> user-space.
>>
>> MPAM has an architectural limit of 2^16 CLOSID values, Intel x86 could
>> be extended beyond 32 values. There is at least one MPAM platform which
>> supports more than 32 CLOSID values.
>>
>> Replace the fixed size bitmap with calls to the bitmap API to allocate
>> an array of a sufficient size.
>>
>> ffs() returns '1' for bit 0, hence the existing code subtracts 1 from
>> the index to get the CLOSID value. find_first_bit() returns the bit
>> number which does not need adjusting.
> 
>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> index 776c8e347654..4e0308040c6e 100644
>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> 
>> @@ -152,20 +152,31 @@ int closids_supported(void)
>>  	return closid_free_map_len;
>>  }
>>  
>> -static void closid_init(void)
>> +static int closid_init(void)
>>  {
>>  	struct resctrl_schema *s;
>> -	u32 rdt_min_closid = 32;
>> +	u32 rdt_min_closid = ~0;
>>  
>>  	/* Compute rdt_min_closid across all resources */
>>  	list_for_each_entry(s, &resctrl_schema_all, list)
>>  		rdt_min_closid = min(rdt_min_closid, s->num_closid);
> 
> Platforms that don't have any controls - only monitors - will still call closid_init().
> Previously this initialised the fixed-sized bitmap, which was harmless as helpers like
> closid_alloc() are never called.
> 
> With this change, rdt_min_closid keeps its dummy initialisation value of ~0, meaning this:
> 
>> -	closid_free_map = BIT_MASK(rdt_min_closid) - 1;
>> +	closid_free_map = bitmap_alloc(rdt_min_closid, GFP_KERNEL);
> 
> Blows up with a greater than 'max order' error.
> 
> I've added a list_empty() check to the top of the function:
> |       /* Monitor only platforms still call closid_init() */
> |       if (list_empty(&resctrl_schema_all))
> |               return 0;
> 
> (list-empty as its clearer what goes wrong without the check).
> I reckon this is minor, so I'll keep the existing tags.
> 
> 
> I'm not aware of anyone building a monitor-only MPAM platform - I configured one by
> accident with one of the software models!
> 

Thank you very much for catching this.

Reinette