mm/fake-numa: fix under-allocation detection in uniform split

When splitting NUMA node uniformly, split_nodes_size_interleave_uniform()
returns the next absolute node ID, not the number of nodes created.

The existing under-allocation detection logic compares next absolute node
ID (ret) and request count (n), which only works when nid starts at 0.

For example, on a system with 2 physical NUMA nodes (node 0: 2GB, node
1: 128MB) and numa=fake=8U, 8 fake nodes are successfully created from
node 0 and split_nodes_size_interleave_uniform() returns 8. For node 1,
fake node nid starts at 8, but only 4 fake nodes are created due to
current FAKE_NODE_MIN_SIZE being 32MB, and
split_nodes_size_interleave_uniform() returns 12. By existing
under-allocation detection logic, "ret < n" (12 < 8) is false, so the
under-allocation will not be detected.

Fix under-allocation detection logic to compare the number of actually
created nodes (ret - nid) against the request count (n). Also skip
under-allocation detection logic for memoryless physical nodes where no
fake nodes are created.

Also, fix the outdated comment describing
split_nodes_size_interleave_uniform() to match the actual return value.

Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Reported-by: Donghyeon Lee <asd142513@gmail.com>
Reported-by: Munhui Chae <mochae@student.42seoul.kr>
Fixes: cc9aec0 ("x86/numa_emulation: Introduce uniform split capability") # 4.19
Link: https://patch.msgid.link/20260417135805.1758378-1-ekffu200098@gmail.com
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Author: hqsz

mm/numa_emulation.c

@@ -214,7 +214,7 @@ static u64 uniform_size(u64 max_addr, u64 base, u64 hole, int nr_nodes)
  * Sets up fake nodes of `size' interleaved over physical nodes ranging from
  * `addr' to `max_addr'.
  *
- * Returns zero on success or negative on error.
+ * Returns node ID of the next node on success or negative error code.
  */
 static int __init split_nodes_size_interleave_uniform(struct numa_meminfo *ei,
 					      struct numa_meminfo *pi,
@@ -398,7 +398,7 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
 	 */
 	if (strchr(emu_cmdline, 'U')) {
 		unsigned long n;
-		int nid = 0;
+		int nid = 0, nr_created;
 
 		n = simple_strtoul(emu_cmdline, &emu_cmdline, 0);
 		ret = -1;
@@ -416,9 +416,18 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
 					n, &pi.blk[0], nid);
 			if (ret < 0)
 				break;
-			if (ret < n) {
+
+			/*
+			 * If no memory was found for this physical node,
+			 * skip the under-allocation check.
+			 */
+			if (ret == nid)
+				continue;
+
+			nr_created = ret - nid;
+			if (nr_created < n) {
 				pr_info("%s: phys: %d only got %d of %ld nodes, failing\n",
-						__func__, i, ret, n);
+						__func__, i, nr_created, n);
 				ret = -1;
 				break;
 			}