[PATCH v2] cgroup/dmem: accept only one region per limit write

[Eric Chanudet <echanude@redhat.com>]

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>
---
Follow up from discussions on a previous thread[1].
If Albert's series[2] lands, I can cleanup and prepare some kunits for
these as well.
[1] https://lore.kernel.org/all/158bc103-7f99-4df4-8d3b-2da9b04ac0ed@lankhorst.se/
[2] https://lore.kernel.org/all/20260519-kunit_cgroups-v4-1-f6c2f498fae4@redhat.com/
---
Changes in v2:
- Handle buf == NULL by testing !buf first after strsep (Natalie)
- Don't allow extra spaces to separate key and value (Natalie)
  Other cgroup files don't (rdma, misc), so stay consistent.
- Link to v1: https://lore.kernel.org/r/20260605-cgroup-dmem-write-single-region-v1-1-9137f296579c@redhat.com
---
 kernel/cgroup/dmem.c | 69 +++++++++++++++++++---------------------------------
 1 file changed, 25 insertions(+), 44 deletions(-)

diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c
index 6430c7ce1e0372f59f1313163fb7630ce49ac1ef..39930c59cb769a505a5852a5644a371fd5596f59 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;
 }

---
base-commit: 640c57d6ca1346a1c2363a3f473b405af979e046
change-id: 20260605-cgroup-dmem-write-single-region-9bf05b6d995d

Best regards,
-- 
Eric Chanudet <echanude@redhat.com>