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

[Jinjie Ruan]

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.
 
 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
-- 
2.34.1