[PATCH v2 6/6] selftests/bpf: add tests for the page_owner bpf_iter target

[Zhen Ni <zhen.ni@easystack.cn>]

Add a sleepable BPF program and a userspace runner covering the seq_file
and open-coded page_owner iterators. The test checks:

  - seq_file walk, unfiltered and pid-filtered: records read back from
    the iterator fd match the lines the program printed.
  - open-coded iterator parity with the seq path.
  - the kfuncs' contracts: a non-empty stack trace, and memcg
    classification.

CONFIG_PAGE_OWNER is enabled in the bpf selftests config so the
program's page_owner BTF types are available. Skipped when debugfs
page_owner is unavailable.

Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
---
Changes in v2:
- Drop the get_nid kfunc coverage.
- Enable CONFIG_PAGE_OWNER in the bpf selftests config.

v1: https://lore.kernel.org/linux-mm/20261009112308.240769-7-zhen.ni@easystack.cn/
---
 tools/testing/selftests/bpf/config            |   2 +
 .../bpf/prog_tests/page_owner_iter.c          | 193 ++++++++++++++++++
 .../selftests/bpf/progs/bpf_iter_page_owner.c | 159 +++++++++++++++
 3 files changed, 354 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/page_owner_iter.c
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_page_owner.c

diff --git a/tools/testing/selftests/bpf/config b/tools/testing/selftests/bpf/config
index ea7044f30adc..731bc3d79881 100644
--- a/tools/testing/selftests/bpf/config
+++ b/tools/testing/selftests/bpf/config
@@ -136,3 +136,5 @@ CONFIG_SMC_HS_CTRL_BPF=y
 CONFIG_DIBS=y
 CONFIG_DIBS_LO=y
 CONFIG_PM_WAKELOCKS=y
+CONFIG_DEBUG_KERNEL=y
+CONFIG_PAGE_OWNER=y
diff --git a/tools/testing/selftests/bpf/prog_tests/page_owner_iter.c b/tools/testing/selftests/bpf/prog_tests/page_owner_iter.c
new file mode 100644
index 000000000000..3684ca308935
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/page_owner_iter.c
@@ -0,0 +1,193 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 */
+
+#include <test_progs.h>
+#include "bpf_iter_page_owner.skel.h"
+
+#define DEBUGFS_PAGE_OWNER	"/sys/kernel/debug/page_owner"
+
+static bool page_owner_available(void)
+{
+	return access(DEBUGFS_PAGE_OWNER, R_OK) == 0;
+}
+
+/* Count link_fd output lines; -1 on error. */
+static long iter_record_count(int link_fd)
+{
+	char *line = NULL;
+	size_t len = 0;
+	long count = 0;
+	int iter_fd;
+	FILE *f;
+
+	iter_fd = bpf_iter_create(link_fd);
+	if (!ASSERT_GE(iter_fd, 0, "bpf_iter_create"))
+		return -1;
+
+	f = fdopen(iter_fd, "r");
+	if (!ASSERT_OK_PTR(f, "fdopen")) {
+		close(iter_fd);
+		return -1;
+	}
+
+	for (;;) {
+		if (getline(&line, &len, f) > 0) {
+			count++;
+			continue;
+		}
+		if (!ferror(f))
+			break;		/* clean EOF */
+		/*
+		 * bpf_seq_read() returns -EAGAIN when a chunk scanned.
+		 * Not an error, just retry the read.
+		 */
+		if (errno == EAGAIN) {
+			clearerr(f);
+			continue;
+		}
+		ASSERT_OK(errno, "iter_read");
+		count = -1;
+		break;
+	}
+
+	free(line);
+	fclose(f);
+	return count;
+}
+
+/* Validate bpf_page_owner_get_memcg_info() results from the open-coded run. */
+static void check_memcg_skel(struct bpf_iter_page_owner *skel)
+{
+	fprintf(stderr,
+		"memcg: seen=%u charged=%u online=%u nodata=%u notsup=%u bad=%u first='%s' name='%s' flags=%u\n",
+		skel->bss->memcg_seen, skel->bss->memcg_charged,
+		skel->bss->memcg_online, skel->bss->memcg_nodata,
+		skel->bss->memcg_notsup, skel->bss->memcg_bad,
+		skel->bss->memcg_first_path, skel->bss->memcg_first_name,
+		skel->bss->memcg_first_flags);
+
+	if (!ASSERT_GT(skel->bss->memcg_seen, 0, "memcg_seen_gt_0"))
+		return;
+
+	/* all -EOPNOTSUPP: CONFIG_MEMCG is off */
+	if (skel->bss->memcg_notsup == skel->bss->memcg_seen) {
+		ASSERT_EQ(skel->bss->memcg_bad, 0, "memcg_stub_invariants");
+		return;
+	}
+
+	if (!ASSERT_EQ(skel->bss->memcg_bad, 0, "memcg_kfunc_invariants"))
+		return;
+
+	/* Every call is either charged or nodata */
+	if (!ASSERT_EQ(skel->bss->memcg_charged + skel->bss->memcg_nodata,
+		       skel->bss->memcg_seen, "memcg_ret_classified"))
+		return;
+
+	if (!skel->bss->memcg_first_saved)
+		return;
+
+	if (!ASSERT_EQ(skel->bss->memcg_first_path[0], '/',
+		       "memcg_first_path_abs"))
+		return;
+
+	if (skel->bss->memcg_first_name[0])
+		ASSERT_OK_PTR(strstr(skel->bss->memcg_first_path,
+				    skel->bss->memcg_first_name),
+			      "memcg_name_in_path");
+}
+
+void serial_test_page_owner_iter(void)
+{
+	struct bpf_iter_page_owner *skel = NULL;
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	long iter_count, unfiltered_count, oc_diff, pid;
+	unsigned int printed_before;
+	int link_fd, prog_fd;
+
+	if (!page_owner_available()) {
+		printf("%s:SKIP:page_owner debugfs unavailable "
+		       "(CONFIG_PAGE_OWNER + page_owner=on)\n", __func__);
+		test__skip();
+		return;
+	}
+
+	skel = bpf_iter_page_owner__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "bpf_iter_page_owner__open_and_load"))
+		return;
+
+	/* --- seq file iterator: unfiltered run --- */
+	skel->bss->filter_en = 0;
+
+	if (!ASSERT_OK(bpf_iter_page_owner__attach(skel), "skel_attach"))
+		goto destroy;
+
+	link_fd = bpf_link__fd(skel->links.dump_page_owner);
+	iter_count = iter_record_count(link_fd);
+	if (iter_count < 0)
+		goto destroy;
+	unfiltered_count = iter_count;
+
+	fprintf(stderr, "iter_count=%ld printed_lines=%u total_seen=%u\n",
+		iter_count, skel->bss->printed_lines, skel->bss->total_seen);
+
+	ASSERT_GT(iter_count, 0, "unfiltered_lines_gt_0");
+
+	ASSERT_EQ((long)skel->bss->printed_lines, iter_count,
+		  "lines_match_printed");
+
+	/* unfiltered: every visited page is printed, so the two must match */
+	ASSERT_EQ(skel->bss->total_seen, skel->bss->printed_lines,
+		  "unfiltered_all_printed");
+
+	/* --- seq file iterator: pid-filtered run --- */
+	pid = getpid();
+	skel->bss->filter_en = 1;
+	skel->bss->filter_pid = (pid_t)pid;
+
+	/* printed_lines is cumulative across runs: compare the delta */
+	printed_before = skel->bss->printed_lines;
+	iter_count = iter_record_count(link_fd);
+	if (iter_count < 0)
+		goto destroy;
+
+	fprintf(stderr,
+		"pid=%ld iter_count=%ld printed_delta=%u\n",
+		pid, iter_count, skel->bss->printed_lines - printed_before);
+
+	ASSERT_GT(iter_count, 0, "filtered_lines_gt_0");
+	ASSERT_EQ(skel->bss->printed_lines - printed_before, iter_count,
+		  "filtered_printed_match");
+
+	/* --- open-coded iterator parity --- */
+	prog_fd = bpf_program__fd(skel->progs.scan_open_coded);
+
+	ASSERT_OK(bpf_prog_test_run_opts(prog_fd, &opts), "prog_test_run");
+	fprintf(stderr,
+		"open_coded_seen=%u nid_seen=%u first_nid=%d "
+		"stack_bytes=%u stack=[\n%s]\n",
+		skel->bss->open_coded_seen, skel->bss->nid_seen,
+		skel->data->first_nid, skel->bss->stack_bytes,
+		skel->bss->stack_text);
+
+	/* same eligibility as the seq path: counts must be close */
+	ASSERT_GT(skel->bss->open_coded_seen, 0, "oc_seen_gt_0");
+	oc_diff = (long)skel->bss->open_coded_seen - unfiltered_count;
+	if (oc_diff < 0)
+		oc_diff = -oc_diff;
+	ASSERT_LE(oc_diff, unfiltered_count / 100 + 64,
+		  "oc_seen_close_to_iter");
+
+	/* the nid kfunc returned a valid node for every page */
+	ASSERT_EQ(skel->bss->nid_seen, skel->bss->open_coded_seen,
+		  "nid_valid_all");
+	ASSERT_GE(skel->data->first_nid, 0, "first_nid_valid");
+
+	ASSERT_GT(skel->bss->stack_bytes, 0, "stack_snprint_ok");
+	ASSERT_OK_PTR(strstr(skel->bss->stack_text, "+0x"), "stack_frames");
+	ASSERT_OK_PTR(strchr(skel->bss->stack_text, '\n'), "stack_lines");
+
+	check_memcg_skel(skel);
+
+destroy:
+	bpf_iter_page_owner__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_page_owner.c b/tools/testing/selftests/bpf/progs/bpf_iter_page_owner.c
new file mode 100644
index 000000000000..be2261b713ea
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/bpf_iter_page_owner.c
@@ -0,0 +1,159 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+__u32 filter_en;
+__u32 filter_pid;
+__u32 total_seen;
+__u32 printed_lines;
+
+__u32 open_coded_seen;
+__u32 nid_seen;
+__s32 first_nid = -1;
+char stack_text[2048];
+__u32 stack_bytes;
+
+__u32 memcg_seen;
+__u32 memcg_charged;
+__u32 memcg_online;
+__u32 memcg_nodata;
+__u32 memcg_notsup;
+__u32 memcg_bad;
+__u32 memcg_first_saved;
+__u32 memcg_first_flags;	/* bit0 is_online, bit1 is_kmem */
+__u32 memcg_first_path_len;
+char memcg_pathbuf[4096];
+char memcg_first_path[4096];
+char memcg_first_name[80];
+
+#define MEMCG_ENODATA 61
+#define MEMCG_EOPNOTSUPP 95
+
+#define MEMCG_F_ONLINE	(1 << 0)
+#define MEMCG_F_OBJCG	(1 << 1)
+
+extern int bpf_iter_page_owner_new(struct bpf_iter_page_owner *it) __ksym;
+extern struct page_owner_scan *
+bpf_iter_page_owner_next(struct bpf_iter_page_owner *it) __ksym;
+extern void bpf_iter_page_owner_destroy(struct bpf_iter_page_owner *it) __ksym;
+extern int bpf_page_owner_stack_snprint(struct page_owner_scan *scan,
+					char *buf__uninit, u32 buf__sz) __ksym;
+extern int bpf_page_owner_get_memcg_info(struct page_owner_scan *scan,
+					 struct memcg_info *info__uninit,
+					 char *buf__uninit, u32 buf__sz) __ksym;
+
+static __u32 copy_str(char *dst, const char *src, const __u32 max)
+{
+	__u32 i = 0;
+
+	while (i + 1 < max && src[i]) {
+		dst[i] = src[i];
+		i++;
+	}
+	dst[i] = '\0';
+	return i;
+}
+
+/*
+ * Exercise bpf_page_owner_get_memcg_info() until 32 pages charged to an
+ * ONLINE memcg have been seen, checking its per-return contract:
+ *  - 0:          buf holds an absolute cgroup path;
+ *  - -ENODATA:   buf was NUL-terminated;
+ *  - -EOPNOTSUPP: counted (CONFIG_MEMCG is off);
+ *
+ * The first online charged sample is saved for userspace to print and
+ * cross-check (name vs basename of path).
+ */
+static void check_memcg(struct page_owner_scan *scan)
+{
+	static struct memcg_info info;
+	int err;
+
+	err = bpf_page_owner_get_memcg_info(scan, &info, memcg_pathbuf,
+					    sizeof(memcg_pathbuf));
+	memcg_seen++;
+
+	if (err == 0) {
+		memcg_charged++;
+
+		if (memcg_pathbuf[0] != '/') {
+			memcg_bad++;
+			return;
+		}
+
+		if (!info.is_online)
+			return;
+		memcg_online++;
+
+		/* keep the first online sample for userspace */
+		if (!memcg_first_saved) {
+			memcg_first_path_len = copy_str(memcg_first_path,
+							memcg_pathbuf,
+							sizeof(memcg_first_path));
+			copy_str(memcg_first_name, info.name,
+				 sizeof(memcg_first_name));
+			memcg_first_flags = MEMCG_F_ONLINE |
+					    (info.is_kmem ? MEMCG_F_OBJCG : 0);
+			memcg_first_saved = 1;
+		}
+	} else if (err == -MEMCG_ENODATA) {
+		memcg_nodata++;
+		if (memcg_pathbuf[0] != '\0')
+			memcg_bad++;
+	} else if (err == -MEMCG_EOPNOTSUPP) {
+		memcg_notsup++;
+		if (memcg_pathbuf[0] != '\0')
+			memcg_bad++;
+	} else {
+		memcg_bad++;
+	}
+}
+
+SEC("syscall")
+int scan_open_coded(const void *ctx)
+{
+	struct bpf_iter_page_owner it;
+	struct page_owner_scan *scan;
+
+	bpf_iter_page_owner_new(&it);
+
+	while ((scan = bpf_iter_page_owner_next(&it))) {
+		open_coded_seen++;
+		if (scan->nid >= 0)
+			nid_seen++;
+		if (memcg_online < 32)
+			check_memcg(scan);
+		/* grab the stack of the 100th page for userspace to check */
+		if (open_coded_seen == 100) {
+			first_nid = scan->nid;
+			if (scan->po_snap.handle)
+				stack_bytes = bpf_page_owner_stack_snprint(
+					scan, stack_text, sizeof(stack_text));
+		}
+	}
+	bpf_iter_page_owner_destroy(&it);
+	return 0;
+}
+
+SEC("iter/page_owner")
+int dump_page_owner(struct bpf_iter__page_owner *ctx)
+{
+	struct page_owner *po = ctx->po;
+
+	if (!po)
+		return 0;
+
+	total_seen++;
+
+	if (filter_en && po->pid != (pid_t)filter_pid)
+		return 0;
+
+	BPF_SEQ_PRINTF(ctx->meta->seq, "pfn %llu pid %d comm %s\n",
+		       ctx->pfn, po->pid, po->comm);
+	printed_lines++;
+	return 0;
+}
-- 
2.20.1