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

[Ben Horgan]

Hi Jinjie,

On 04/11/2026 10:19, Jinjie Ruan wrote:
> From: Peter Newman <peternewman@xxxxxxxxxx>
> 
> Renaming a monitor group to move it below a different control group
> changes the CLOSID of its tasks. Where the RMID only has meaning
> together with the CLOSID, the RMID index changes as well: on arm64 MPAM
> the RMID is a PMG that extends the PARTID, so the group's counters
> belong to the old PARTID and cannot follow the move. The monitor data
> would be reset and would then report the new PARTID's traffic.
> 
> Check that the RMID index is unchanged before moving the group and
> reject the rename with -EOPNOTSUPP otherwise. Only MPAM systems are
> affected; architectures where the RMID is independent of the CLOSID,
> such as x86, keep the same index and still allow the move. Document
> the restriction in resctrl.rst.
> 
> Fixes: 3e9b35823aab ("arm_mpam: resctrl: Add rmid index helpers")
> Suggested-by: Ben Horgan <ben.horgan@xxxxxxx>

Perhaps stretching the meaning of a 'Suggested-by' tag as I only suggested the repost of Peter's patch.

It is probably worth pointing out that the moving of monitor groups is existingly broken other than
just the resetting of mbm_total_bytes counters and so the current behaviour isn't something that an
existing user could be relying on. As you already point out in [1]

[1] https://lore.kernel.org/lkml/20261031101052.1692230-1-ruanjinjie@xxxxxxxxxx/

Thanks,

Ben

> 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/
> ---
>  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..5f21ab20030a 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, such as
> +arm64 MPAM. No other move operation is currently allowed other than
> +simply renaming a CTRL_MON or MON group.
>  
>  All groups contain the following files:
>  
> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index 9290a346386e..358f2dc8301d 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
> @@ -4405,6 +4405,19 @@ 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