[PATCH 03/16] selftests/mm: add folio-order detection self-check

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

The upcoming khugepaged mTHP tests detect collapse results with the
vm_util folio-order helpers instead of smaps AnonHugePages, which only
sees PMD mappings. Before any collapse test trusts those helpers, make
sure they agree with the kernel about what backs a mapping.

For every anon THP order the kernel supports, fault memory in with
only that order enabled and require the helpers to classify the
backing as exactly that order: not a neighbouring order, and 4K-backed
memory as order 0.

Runs in the thp category of run_vmtests.sh. Verified on x86-64 4K
(orders 0, 2-9) and arm64 64K (orders 0, 2-13).

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 tools/testing/selftests/mm/.gitignore         |   1 +
 tools/testing/selftests/mm/Makefile           |   1 +
 .../testing/selftests/mm/folio_order_check.c  | 138 ++++++++++++++++++
 tools/testing/selftests/mm/run_vmtests.sh     |   2 +
 4 files changed, 142 insertions(+)
 create mode 100644 tools/testing/selftests/mm/folio_order_check.c

diff --git a/tools/testing/selftests/mm/.gitignore b/tools/testing/selftests/mm/.gitignore
index 9ccd9e1447e6..54eefd8f97bc 100644
--- a/tools/testing/selftests/mm/.gitignore
+++ b/tools/testing/selftests/mm/.gitignore
@@ -66,3 +66,4 @@ merge
 prctl_thp_disable
 rmap
 folio_split_race_test
+folio_order_check
diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index 25d10ced3b3b..33917c76b871 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -104,6 +104,7 @@ TEST_GEN_FILES += guard-regions
 TEST_GEN_FILES += merge
 TEST_GEN_FILES += rmap
 TEST_GEN_FILES += folio_split_race_test
+TEST_GEN_FILES += folio_order_check
 
 ifneq ($(ARCH),arm64)
 TEST_GEN_FILES += soft-dirty
diff --git a/tools/testing/selftests/mm/folio_order_check.c b/tools/testing/selftests/mm/folio_order_check.c
new file mode 100644
index 000000000000..f70d766ff3eb
--- /dev/null
+++ b/tools/testing/selftests/mm/folio_order_check.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Self-check for the vm_util folio-order detection helpers,
+ * is_backed_by_folio() and is_range_backed_by_folio_orders().
+ *
+ * For every anon THP order the kernel supports, fault memory in with only
+ * that order enabled and verify the helpers report exactly that order:
+ * not a neighbouring order, and plain 4K memory as order 0. The helpers
+ * are what the khugepaged mTHP tests use to detect collapse results, so
+ * they must agree with the kernel's own idea of the backing before any
+ * collapse test relies on them.
+ */
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#include "kselftest.h"
+#include "vm_util.h"
+#include "hugepage_settings.h"
+
+static int pagemap_fd;
+static int kpageflags_fd;
+
+/* mmap an anon VMA of exactly @size bytes at a @size-aligned address. */
+static char *alloc_aligned(size_t size)
+{
+	size_t len = size * 2;
+	uintptr_t aligned;
+	char *p;
+
+	p = mmap(NULL, len, PROT_READ | PROT_WRITE,
+		 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
+	if (p == MAP_FAILED)
+		ksft_exit_fail_perror("mmap()");
+
+	aligned = ((uintptr_t)p + size - 1) & ~(size - 1);
+	if (aligned != (uintptr_t)p)
+		munmap(p, aligned - (uintptr_t)p);
+	if (aligned + size != (uintptr_t)p + len)
+		munmap((char *)aligned + size,
+		       (uintptr_t)p + len - aligned - size);
+
+	return (char *)aligned;
+}
+
+/*
+ * Enable only @order (order 0: nothing), fault one aligned window in and
+ * check the helpers see exactly @order.
+ */
+static void check_order(int order)
+{
+	struct thp_settings settings = *thp_current_settings();
+	size_t size = psize() << order;
+	bool ok = true;
+	char *p;
+	int i;
+
+	for (i = 0; i < NR_ORDERS; i++)
+		settings.hugepages[i].enabled = THP_NEVER;
+	if (order)
+		settings.hugepages[order].enabled = THP_ALWAYS;
+	thp_push_settings(&settings);
+
+	p = alloc_aligned(size);
+	*p = 1;
+
+	if (!is_range_backed_by_folio_orders(p, size, order,
+					     pagemap_fd, kpageflags_fd)) {
+		ksft_print_msg("order %d not detected after fault\n", order);
+		ok = false;
+	}
+
+	/* A lower order must be rejected: the folio is larger. */
+	if (order && is_range_backed_by_folio_orders(p, size, order - 1,
+						     pagemap_fd,
+						     kpageflags_fd)) {
+		ksft_print_msg("order %d also reported as order %d\n",
+			       order, order - 1);
+		ok = false;
+	}
+
+	/* Order 0 pages must not look like any large folio, and vice versa. */
+	if (order && is_range_backed_by_folio_orders(p, size, 0,
+						     pagemap_fd,
+						     kpageflags_fd)) {
+		ksft_print_msg("order %d also reported as order 0\n", order);
+		ok = false;
+	}
+
+	munmap(p, size);
+	thp_pop_settings();
+
+	ksft_test_result(ok, "order %d classified\n", order);
+}
+
+int main(void)
+{
+	struct thp_settings settings;
+	unsigned long orders;
+	int order;
+
+	ksft_print_header();
+
+	if (!thp_available())
+		ksft_exit_skip("Transparent Hugepages not available\n");
+
+	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+	if (pagemap_fd < 0)
+		ksft_exit_fail_perror("open(\"/proc/self/pagemap\")");
+	kpageflags_fd = open("/proc/kpageflags", O_RDONLY);
+	if (kpageflags_fd < 0)
+		ksft_exit_skip("open(\"/proc/kpageflags\") requires root\n");
+
+	orders = thp_supported_orders();
+	if (!orders)
+		ksft_exit_skip("No supported THP orders\n");
+
+	ksft_set_plan(__builtin_popcountl(orders) + 1);
+
+	thp_save_settings();
+	thp_read_settings(&settings);
+	/* Base of the settings stack; the bottom entry is never popped. */
+	thp_push_settings(&settings);
+
+	check_order(0);
+	for (order = 1; order < NR_ORDERS; order++) {
+		if (!(orders & (1UL << order)))
+			continue;
+		check_order(order);
+	}
+
+	thp_restore_settings();
+
+	ksft_finished();
+}
diff --git a/tools/testing/selftests/mm/run_vmtests.sh b/tools/testing/selftests/mm/run_vmtests.sh
index 687d115e3bd8..8bf898b71350 100755
--- a/tools/testing/selftests/mm/run_vmtests.sh
+++ b/tools/testing/selftests/mm/run_vmtests.sh
@@ -402,6 +402,8 @@ CATEGORY="pfnmap" run_test ./pfnmap
 # COW tests
 CATEGORY="cow" run_test ./cow
 
+CATEGORY="thp" run_test ./folio_order_check
+
 CATEGORY="thp" run_test ./khugepaged
 
 CATEGORY="thp" run_test ./khugepaged -s 2
-- 
2.54.0