cgroup/dmem: accept only one region per limit write

Accept only one "region value" pair entry for the dmem.max, dmem.min,
dmem.low files.

This changes the UAPI that otherwise accepted multiple lines for setting
multiple entries in one write. No existing user is known to rely on
writing multiple regions in a single write.

Processing multiple regions in dmemcg_limit_write() could quietly change
first limits before failing on a later one and returning an error to the
writer, with no indication some changes occurred.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Eric Chanudet <echanude@redhat.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Tested-by: Thadeu Lima de Souza Cascardo <cascardo@igalia.com>
Reviewed-by: Natalie Vock <natalie.vock@gmx.de>
Link: https://patch.msgid.link/20260608-cgroup-dmem-write-single-region-v2-1-b0cd6c4ccf1b@redhat.com
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>

1 files changed, 25 insertions, 44 deletions

diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
index 6430c7ce1e03..39930c59cb76 100644
--- a/kernel/cgroup/dmem.c
+++ b/kernel/cgroup/dmem.c
@@ -734,57 +734,38 @@ static ssize_t dmemcg_limit_write(struct kernfs_open_file *of,
 				 void (*apply)(struct dmem_cgroup_pool_state *, u64))
 {
 	struct dmemcg_state *dmemcs = css_to_dmemcs(of_css(of));
-	int err = 0;
-
-	while (buf && !err) {
-		struct dmem_cgroup_pool_state *pool = NULL;
-		char *options, *region_name;
-		struct dmem_cgroup_region *region;
-		u64 new_limit;
-
-		options = buf;
-		buf = strchr(buf, '\n');
-		if (buf)
-			*buf++ = '\0';
-
-		options = strstrip(options);
-
-		/* eat empty lines */
-		if (!options[0])
-			continue;
-
-		region_name = strsep(&options, " \t");
-		if (!region_name[0])
-			continue;
-
-		if (!options || !*options)
-			return -EINVAL;
+	struct dmem_cgroup_pool_state *pool;
+	struct dmem_cgroup_region *region;
+	char *region_name;
+	u64 new_limit;
+	int err;
 
-		rcu_read_lock();
-		region = dmemcg_get_region_by_name(region_name);
-		rcu_read_unlock();
+	buf = strstrip(buf);
+	region_name = strsep(&buf, " \t");
+	if (!buf || !region_name[0])
+		return -EINVAL;
 
-		if (!region)
-			return -EINVAL;
+	rcu_read_lock();
+	region = dmemcg_get_region_by_name(region_name);
+	rcu_read_unlock();
+	if (!region)
+		return -EINVAL;
 
-		err = dmemcg_parse_limit(options, &new_limit);
-		if (err < 0)
-			goto out_put;
+	err = dmemcg_parse_limit(buf, &new_limit);
+	if (err < 0)
+		goto out_put;
 
-		pool = get_cg_pool_unlocked(dmemcs, region);
-		if (IS_ERR(pool)) {
-			err = PTR_ERR(pool);
-			goto out_put;
-		}
+	pool = get_cg_pool_unlocked(dmemcs, region);
+	if (IS_ERR(pool)) {
+		err = PTR_ERR(pool);
+		goto out_put;
+	}
 
-		/* And commit */
-		apply(pool, new_limit);
-		dmemcg_pool_put(pool);
+	apply(pool, new_limit);
+	dmemcg_pool_put(pool);
 
 out_put:
-		kref_put(&region->ref, dmemcg_free_region);
-	}
-
+	kref_put(&region->ref, dmemcg_free_region);
 
 	return err ?: nbytes;
 }