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

[Jinjie Ruan]

在 2026/11/10 3:22, Reinette Chatre 写道:
> Hi Jinjie,

Hi Reinette，

> 
> Thank you very much for picking this fix up.
> 
> For the subject, since the RMID actually never changes, could it instead be:
> 	fs/resctrl: Disallow monitor group rename when the RMID index changes
> 
> On 11/5/26 4:35 AM, Jinjie Ruan wrote:
>> From: Peter Newman <peternewman@xxxxxxxxxx>
>>
>> A monitor group can be renamed into the mon_groups directory of another
> 
> "renamed" -> "moved"?

Ok.

> 
> (Since this patch would still support rename of MPAM monitor groups.)

Right!

> 
>> 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.
> 
> Does this not also mean that the the monitor group is assigned an RMID
> index that is never allocated to it? Sounds like this would cause issues
> when the monitor group is removed and resctrl tries to free that RMID.

Yes, will mention it.

> 
>>
>> - 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.
> 
> Could this snippet please instead motivate why the issues with monitor
> group move are not/cannot be fixed to help explain why this reject
> is needed instead?

Sure.

> 
>>
>> 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
> 
> Looks like 3e9b35823aab was added in 7.1 so the range is not needed?
> 
>> Link: https://lore.kernel.org/lkml/20241205153845.394714-1-peternewman@xxxxxxxxxx/
>> Link: https://lore.kernel.org/lkml/20261031101052.1692230-1-ruanjinjie@xxxxxxxxxx/
> 
> The changelog contains no references to these Link: entries? What are they for?
> Looks like first is an earlier version of the patch so does not look to be needed
> as part of permanent changelog related to this patch?

Will remove it. Thank you!

> 
> Please see: "Ordering of commit tags" in Documentation/process/maintainer-tip.rst
> on how changelog can refer to Link: tags.
> 
> The second Link: seems to be pointing to the decision that this user space request
> needs to be rejected instead of fixed. Looks like the relevant message is
> 
> 	https://lore.kernel.org/lkml/3a637631-c8ec-4e26-a03c-5ac80d16fab3@xxxxxxx/
> 
> you could use similar language as recent commit 
> 	7782a6a8dfc0 ("x86,fs/resctrl: Keep mbm_assign_mode in default mode at boot")
> 
> That points to the decision in a "Link:" tag and then uses text in changelog like:
> 	"... as decided in the thread at the Link tag." 

OK

> 
> Above is not actually what Documentation/process/maintainer-tip.rst documents but it
> is what x86 maintainer changed it to after the original commit followed
> Documentation/process/maintainer-tip.rst, so I guess either may be ok.
> 
> 
>> ---
> 
> 
>> ---
>>  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.
> 
> merging the discussion in 
>  https://lore.kernel.org/lkml/113c13ba-e1de-44b1-9a6f-4bd94ecbd71e@xxxxxxx/
> 
> To avoid user needing to understand the RMID dependency, how about:
> 
> ---8<---
> @@ -566,10 +566,10 @@ 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.
> +without impacting its monitoring data or assigned tasks. This operation is
> +not allowed for MON groups which monitor CPUs, or when the monitoring data
> +or assigned tasks would be impacted. No other move operation is currently
> +allowed other than simply renaming a CTRL_MON or MON group.

Will update it.

>  
>  All groups contain the following files:
> ---8<---
> 
>>  
>>  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() &&
> 
> The above two checks look unnecessary:
> 	"rdtgrp->mon.parent != new_prdtgrp" - this check is used to ensure that
> 	these systems can still just rename existing monitor groups. In this scenario
> 	the CLOSID will be compared against itself and the two encode() calls will
> 	return the same index, no?

Yes, the resctrl_arch_rmid_idx_encode() will equal, this is redundant。

> 
> 	"resctrl_arch_mon_capable()" - earlier in function the source is required
> 	to be a MON group, which can only be possible if the system supports
> 	monitoring.

Got it.

> 
>> +	    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");
> 
> Could this use same terminology as resctrl.rst to help user match documented
> behavior with the error message? For example,
> 
> 		"Moving a MON group to a new CTRL_MON group is not supported\n"

Sure.

> 		
> 
>> +		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
> 
> Reinette

-- 
Best regards,
Jinjie