Re: [PATCH v2] fs/resctrl: Disallow monitor group rename when the RMID changes

[Ben Horgan <ben.horgan@arm.com>]

Hi Jinjie,

On 09/11/2026 10:52, Ben Horgan wrote:
> Hi Jinjie,
> 
> On 05/11/2026 12:35, Jinjie Ruan wrote:
>> From: Peter Newman <peternewman@google.com>
>>
>> A monitor group can be renamed into the mon_groups directory of another
>> control group to move its tasks to that control group's allocation
>> while keeping their monitoring data. Where the RMID is only meaningful
>> together with the CLOSID, as on arm64 MPAM where the RMID is a PMG that
>> extends the PARTID, this move does not work. The rename succeeds, but:
>>
>> - The group's monitoring data is reset. Its counters are identified by
>>   the CLOSID and RMID together, so after the move the group reads the
>>   counters of the new CLOSID instead of the ones its tasks used.
>>
>> - The tasks are not moved. mongrp_reparent() sets the new parent and
>>   CLOSID of the group before rdt_move_group_tasks() looks for the tasks
>>   to update, and the search then matches on the new CLOSID, which none
>>   of the tasks carries yet. The tasks keep the old CLOSID and the old
>>   control group's allocation, and the group does not count them.
>>
>> The move has never worked on these systems, so rejecting it does not
>> take away behavior that an existing user can rely on.
>>
>> Reject the move with -EOPNOTSUPP when it changes the RMID index of the
>> group, and document the restriction in resctrl.rst. Where the RMID does
>> not depend on the CLOSID, such as on x86, the move keeps the same index
>> and is still allowed.
>>
>> Fixes: 3e9b35823aab ("arm_mpam: resctrl: Add rmid index helpers")
>> Signed-off-by: Peter Newman <peternewman@google.com>
>> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
>> Cc: <stable@vger.kernel.org> # 7.1.x
>> Link: https://lore.kernel.org/lkml/20241205153845.394714-1-peternewman@google.com/
>> Link: https://lore.kernel.org/lkml/20261031101052.1692230-1-ruanjinjie@huawei.com/
>> ---
>> v2:
>> - Drop the Suggested-by tag.
>> - Describe the systems by whether the RMID depends on the CLOSID rather
>>   than by MPAM, in both the changelog and resctrl.rst.
>> - Explain that moving a monitor group is already broken on these systems
>>   because the tasks are not moved, so no existing user relies on it.
>> - v1: https://lore.kernel.org/all/20261104101913.586595-1-ruanjinjie@huawei.com/
>> ---
>>  Documentation/filesystems/resctrl.rst |  7 ++++---
>>  fs/resctrl/rdtgroup.c                 | 13 +++++++++++++
>>  2 files changed, 17 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
>> index b52795e03303..9ae34aace0ff 100644
>> --- a/Documentation/filesystems/resctrl.rst
>> +++ b/Documentation/filesystems/resctrl.rst
>> @@ -552,9 +552,10 @@ will automatically remove all MON groups below it.
>>  Moving MON group directories to a new parent CTRL_MON group is supported
>>  for the purpose of changing the resource allocations of a MON group
>>  without impacting its monitoring data or assigned tasks. This operation
>> -is not allowed for MON groups which monitor CPUs. No other move
>> -operation is currently allowed other than simply renaming a CTRL_MON or
>> -MON group.
>> +is not allowed for MON groups which monitor CPUs, and fails with
>> +-EOPNOTSUPP on systems where the RMID depends on the CLOSID. No other
>> +move operation is currently allowed other than simply renaming a
>> +CTRL_MON or MON group.
> 
> A user may not readily know whether RMID depends on CLOSID. To document
> this properly we need something specific we can point the user too.
> Perhaps this can be a file in the info directory specifying whether the
> move is possible or not, info/moveable_mon_group, as we discussed in v1.
> I suggest a precursor patch adding this file, followed by this patch
> which can then refer to the file.

On the other hand, that does add an extra dependency for any backporting
of this fix.

Thanks,

Ben

> 
> Thanks,
> 
> Ben
> 
>>  
>>  All groups contain the following files:
>>  
>> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
>> index e6b64a5c9302..3c002b711024 100644
>> --- a/fs/resctrl/rdtgroup.c
>> +++ b/fs/resctrl/rdtgroup.c
>> @@ -4398,6 +4398,19 @@ static int rdtgroup_rename(struct kernfs_node *kn,
>>  		goto out;
>>  	}
>>  
>> +	/*
>> +	 * Moving the group changes the CLOSID. Where the RMID is only
>> +	 * meaningful together with the CLOSID, the RMID index changes and the
>> +	 * monitor state cannot follow the move.
>> +	 */
>> +	if (rdtgrp->mon.parent != new_prdtgrp && resctrl_arch_mon_capable() &&
>> +	    resctrl_arch_rmid_idx_encode(rdtgrp->closid, rdtgrp->mon.rmid) !=
>> +	    resctrl_arch_rmid_idx_encode(new_prdtgrp->closid, rdtgrp->mon.rmid)) {
>> +		rdt_last_cmd_puts("Changing parent control group not supported\n");
>> +		ret = -EOPNOTSUPP;
>> +		goto out;
>> +	}
>> +
>>  	/*
>>  	 * If the MON group is monitoring CPUs, the CPUs must be assigned to the
>>  	 * current parent CTRL_MON group and therefore cannot be assigned to
>