[PATCH 2/2] selftests/mm: add test for vrealloc() shrink page freeing

[Jill Ravaliya <jillravaliya@gmail.com>]

Add a selftest that verifies vrealloc() frees physical pages
when shrinking an allocation.

The test loads a kernel module that:
1. Allocates 10MB with vmalloc()
2. Touches all pages to force physical allocation
3. Shrinks to 2MB with vrealloc()
4. Verifies free page count increased after shrink

Without the fix, the test fails because no pages are freed.
With the fix applied, the test passes confirming ~2048 pages
are returned to the system after shrinking from 10MB to 2MB.

Tested on kernel 7.0.0 in QEMU.

Signed-off-by: Jill Ravaliya <jillravaliya@gmail.com>
---
 tools/testing/selftests/mm/Makefile           |  5 ++
 .../selftests/mm/vrealloc_shrink_test.c       | 65 +++++++++++++++++++
 2 files changed, 70 insertions(+)
 create mode 100644 tools/testing/selftests/mm/vrealloc_shrink_test.c

diff --git a/tools/testing/selftests/mm/Makefile b/tools/testing/selftests/mm/Makefile
index cd24596cd..4eab7c76c 100644
--- a/tools/testing/selftests/mm/Makefile
+++ b/tools/testing/selftests/mm/Makefile
@@ -104,6 +104,7 @@ TEST_GEN_FILES += hugetlb_dio
 TEST_GEN_FILES += droppable
 TEST_GEN_FILES += guard-regions
 TEST_GEN_FILES += merge
+TEST_GEN_FILES += vrealloc_shrink_test
 TEST_GEN_FILES += rmap
 TEST_GEN_FILES += folio_split_race_test
 
@@ -282,3 +283,7 @@ warn_missing_page_frag:
 	echo "Warning: $(PAGE_FRAG_WARNING). page_frag test will be skipped." ; \
 	echo
 endif
+
+# vrealloc shrink test module
+vrealloc_shrink_mod.ko: vrealloc_shrink_mod.c
+	$(MAKE) -C $(KDIR) M=$(PWD) modules
diff --git a/tools/testing/selftests/mm/vrealloc_shrink_test.c b/tools/testing/selftests/mm/vrealloc_shrink_test.c
new file mode 100644
index 000000000..cf4263074
--- /dev/null
+++ b/tools/testing/selftests/mm/vrealloc_shrink_test.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test that vrealloc() frees physical pages when shrinking.
+ *
+ * vrealloc() shrink path previously zeroed unused memory and updated
+ * vm->requested_size, but never freed the physical pages backing the
+ * unused portion of the allocation. This test verifies the fix by
+ * loading a kernel module that directly measures nr_pages before and
+ * after vrealloc() shrink.
+ *
+ * Copyright (C) 2026 Jill Ravaliya
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include "../kselftest.h"
+
+#define MODULE_NAME "vrealloc_shrink_mod"
+#define DMESG_PASS  "vrealloc_shrink: PASS"
+#define DMESG_FAIL  "vrealloc_shrink: FAIL"
+
+static int run_cmd(const char *cmd)
+{
+	return system(cmd);
+}
+
+static int check_dmesg_for(const char *pattern)
+{
+	char cmd[256];
+	snprintf(cmd, sizeof(cmd),
+		 "dmesg | grep -q '%s'", pattern);
+	return system(cmd) == 0;
+}
+
+int main(void)
+{
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	/* Insert the test module */
+	if (run_cmd("insmod " MODULE_NAME ".ko") != 0) {
+		ksft_test_result_skip(
+			"could not load %s.ko - is it built?\n",
+			MODULE_NAME);
+		ksft_finished();
+	}
+
+	/* Check dmesg for pass/fail */
+	if (check_dmesg_for(DMESG_PASS)) {
+		ksft_test_result_pass(
+			"vrealloc shrink frees physical pages\n");
+	} else if (check_dmesg_for(DMESG_FAIL)) {
+		ksft_test_result_fail(
+			"vrealloc shrink did NOT free physical pages\n");
+	} else {
+		ksft_test_result_fail(
+			"could not find test result in dmesg\n");
+	}
+
+	run_cmd("rmmod " MODULE_NAME);
+	ksft_finished();
+}
-- 
2.43.0