info/last_cmd_status is intended to contain more information if a write to
any resctrl file fails. Writes to max_threshold_occupancy did not receive
last_cmd_status support during initial last_cmd_status enabling. Add it now.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
Changes since v1:
- Add Ben's RB tag.

Changes since v2:
- Switch to guard(). (Chenyu)
- Drop Ben's RB tag due to significant code change.
---
 fs/resctrl/rdtgroup.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 5dfdaa6f9d8f..5e3ea92cc243 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -12,6 +12,7 @@
 
 #define pr_fmt(fmt)	KBUILD_MODNAME ": " fmt
 
+#include <linux/cleanup.h>
 #include <linux/cpu.h>
 #include <linux/debugfs.h>
 #include <linux/fs.h>
@@ -1238,12 +1239,20 @@ static ssize_t max_threshold_occ_write(struct kernfs_open_file *of,
 	unsigned int bytes;
 	int ret;
 
+	guard(mutex)(&rdtgroup_mutex);
+	rdt_last_cmd_clear();
+
 	ret = kstrtouint(buf, 0, &bytes);
-	if (ret)
+	if (ret) {
+		rdt_last_cmd_puts("Invalid input\n");
 		return ret;
+	}
 
-	if (bytes > resctrl_rmid_realloc_limit)
+	if (bytes > resctrl_rmid_realloc_limit) {
+		rdt_last_cmd_printf("Exceeds limit (before adjustment) of %u bytes\n",
+				    resctrl_rmid_realloc_limit);
 		return -EINVAL;
+	}
 
 	resctrl_rmid_realloc_threshold = resctrl_arch_round_mon_val(bytes);
 
-- 
2.50.1