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

[Reinette Chatre]

Hi Ben,

On 11/9/26 9:47 AM, Ben Horgan wrote:
> On 09/11/2026 17:40, Reinette Chatre wrote:
>> 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@xxxxxxxxxx>
>>>>>
>>>>> 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@xxxxxxxxxx>
>>>>> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
>>>>> Cc: <stable@xxxxxxxxxxxxxxx> # 7.1.x
>>>>> Link: https://lore.kernel.org/lkml/20241205153845.394714-1-peternewman@xxxxxxxxxx/
>>>>> Link: https://lore.kernel.org/lkml/20261031101052.1692230-1-ruanjinjie@xxxxxxxxxx/
>>>>> ---
>>>>> 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@xxxxxxxxxx/
>>>>> ---
>>>>>  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.
> 
> Not strictly, no. Up to a point the documentation does but we could go
> back to the documentation in v1 which mentions MPAM for the fix and
> then, as a new feature, add the new interface on top of that. The
> documentation can then be updated to mention the new interface only at
> the time the new interface is added.
Existing documentation makes a promise that monitoring data is not
impacted by a move. The fix could start by just updating that
snippet to match how other failures are handled. To keep things together
I'll combine a suggestion in this regard with my other feedback in response
to the patch self.

Reinette