Return -EBUSY to userspace when writing dmem.max below the cgroup's
current device memory usage, rather than silently leaving the limit
unchanged.

Assisted-by: GitHub Copilot:claude-sonnet-4.6
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
---
 kernel/cgroup/dmem.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
index 9d95824dc6fa..3e6d4c0b26a1 100644
--- a/kernel/cgroup/dmem.c
+++ b/kernel/cgroup/dmem.c
@@ -144,22 +144,24 @@ static void free_cg_pool(struct dmem_cgroup_pool_state *pool)
 	dmemcg_pool_put(pool);
 }
 
-static void
+static int
 set_resource_min(struct dmem_cgroup_pool_state *pool, u64 val)
 {
 	page_counter_set_min(&pool->cnt, val);
+	return 0;
 }
 
-static void
+static int
 set_resource_low(struct dmem_cgroup_pool_state *pool, u64 val)
 {
 	page_counter_set_low(&pool->cnt, val);
+	return 0;
 }
 
-static void
+static int
 set_resource_max(struct dmem_cgroup_pool_state *pool, u64 val)
 {
-	page_counter_set_max(&pool->cnt, val);
+	return page_counter_set_max(&pool->cnt, val);
 }
 
 static u64 get_resource_low(struct dmem_cgroup_pool_state *pool)
@@ -726,7 +728,7 @@ static int dmemcg_parse_limit(char *options, struct dmem_cgroup_region *region,
 
 static ssize_t dmemcg_limit_write(struct kernfs_open_file *of,
 				 char *buf, size_t nbytes, loff_t off,
-				 void (*apply)(struct dmem_cgroup_pool_state *, u64))
+				 int (*apply)(struct dmem_cgroup_pool_state *, u64))
 {
 	struct dmemcg_state *dmemcs = css_to_dmemcs(of_css(of));
 	int err = 0;
@@ -773,7 +775,7 @@ static ssize_t dmemcg_limit_write(struct kernfs_open_file *of,
 		}
 
 		/* And commit */
-		apply(pool, new_limit);
+		err = apply(pool, new_limit);
 		dmemcg_pool_put(pool);
 
 out_put:
-- 
2.53.0