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

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

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.

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