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

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Ben,

On 11/9/26 3:03 AM, Ben Horgan wrote:
> On 09/11/2026 10:52, Ben Horgan wrote:
>> 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.
Does this fix really depend on a new user interface? 

I understand this is helpful to user space and this can be added, but I do
not see how this fix requires it.

Reinette