linux-kernel - [PATCH 1/3] tools/testing/selftests: add mremap() shrink test for multiple VMAs

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Message-ID: <f08122893a26092a2bec6e69443e87f468ffdbed.1753119043.git.lorenzo.stoakes@oracle.com>
Date: Mon, 21 Jul 2025 18:33:25 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] tools/testing/selftests: add mremap() shrink test for multiple VMAs

There is a an apparently little-known feature of mremap() whereby, in stark
contrast to other modes (other than the recently introduced capacity to
move multiple VMAs), the input source range span multiple VMAs with gaps
between.

This is, when shrinking a VMA, whether moving it or not, and the shrink
would reduce the range to a single VMA - this is permitted, as the shrink
is actioned by an unmap.

This patch adds tests to assert that this behaves as expected.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 tools/testing/selftests/mm/mremap_test.c | 83 +++++++++++++++++++++++-
 1 file changed, 82 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/mremap_test.c b/tools/testing/selftests/mm/mremap_test.c
index 0a49be11e614..141a9032414e 100644
--- a/tools/testing/selftests/mm/mremap_test.c
+++ b/tools/testing/selftests/mm/mremap_test.c
@@ -523,6 +523,85 @@ static void mremap_move_multiple_vmas(unsigned int pattern_seed,
 		ksft_test_result_fail("%s\n", test_name);
 }
 
+static void mremap_shrink_multiple_vmas(unsigned long page_size,
+					bool inplace)
+{
+	char *test_name = "mremap shrink multiple vmas";
+	const size_t size = 10 * page_size;
+	bool success = true;
+	char *ptr, *tgt_ptr;
+	void *res;
+	int i;
+
+	ptr = mmap(NULL, size, PROT_READ | PROT_WRITE,
+		   MAP_PRIVATE | MAP_ANON, -1, 0);
+	if (ptr == MAP_FAILED) {
+		perror("mmap");
+		success = false;
+		goto out;
+	}
+
+	tgt_ptr = mmap(NULL, size, PROT_READ | PROT_WRITE,
+		       MAP_PRIVATE | MAP_ANON, -1, 0);
+	if (tgt_ptr == MAP_FAILED) {
+		perror("mmap");
+		success = false;
+		goto out;
+	}
+	if (munmap(tgt_ptr, size)) {
+		perror("munmap");
+		success = false;
+		goto out_unmap;
+	}
+
+	/*
+	 * Unmap so we end up with:
+	 *
+	 *  0   2   4   6   8   10 offset in buffer
+	 * |*| |*| |*| |*| |*| |*|
+	 * |*| |*| |*| |*| |*| |*|
+	 */
+	for (i = 1; i < 10; i += 2) {
+		if (munmap(&ptr[i * page_size], page_size)) {
+			perror("munmap");
+			success = false;
+			goto out_unmap;
+		}
+	}
+
+	/*
+	 * Shrink in-place across multiple VMAs and gaps so we end up with:
+	 *
+	 *  0
+	 * |*|
+	 * |*|
+	 */
+	if (inplace)
+		res = mremap(ptr, size, page_size, 0);
+	else
+		res = mremap(ptr, size, page_size, MREMAP_MAYMOVE | MREMAP_FIXED,
+			     tgt_ptr);
+
+	if (res == MAP_FAILED) {
+		perror("mremap");
+		success = false;
+		goto out_unmap;
+	}
+
+out_unmap:
+	if (munmap(tgt_ptr, size))
+		perror("munmap tgt");
+	if (munmap(ptr, size))
+		perror("munmap src");
+out:
+	if (success)
+		ksft_test_result_pass("%s%s\n", test_name,
+				      inplace ? " [inplace]" : "");
+	else
+		ksft_test_result_fail("%s%s\n", test_name,
+				      inplace ? " [inplace]" : "");
+}
+
 /* Returns the time taken for the remap on success else returns -1. */
 static long long remap_region(struct config c, unsigned int threshold_mb,
 			      char *rand_addr)
@@ -864,7 +943,7 @@ int main(int argc, char **argv)
 	char *rand_addr;
 	size_t rand_size;
 	int num_expand_tests = 2;
-	int num_misc_tests = 3;
+	int num_misc_tests = 5;
 	struct test test_cases[MAX_TEST] = {};
 	struct test perf_test_cases[MAX_PERF_TEST];
 	int page_size;
@@ -992,6 +1071,8 @@ int main(int argc, char **argv)
 	mremap_move_within_range(pattern_seed, rand_addr);
 	mremap_move_1mb_from_start(pattern_seed, rand_addr);
 	mremap_move_multiple_vmas(pattern_seed, page_size);
+	mremap_shrink_multiple_vmas(page_size, /* inplace= */true);
+	mremap_shrink_multiple_vmas(page_size, /* inplace= */false);
 
 	if (run_perf_tests) {
 		ksft_print_msg("\n%s\n",
-- 
2.50.1