linux-kernel - [PATCH 2/2] testing/selftests/mm: add soft-dirty merge self-test

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Message-ID: <0db6aeb73d42876756ba73163cc0cc6e7e8c6100.1763142412.git.lorenzo.stoakes@oracle.com>
Date: Fri, 14 Nov 2025 17:53:19 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...nel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] testing/selftests/mm: add soft-dirty merge self-test

Assert that we correctly merge VMAs containing VM_SOFTDIRTY flags now that
we correctly handle these as sticky.

In order to do so, we have to account for the fact the pagemap interface
checks soft dirty PTEs and additionally that newly merged VMAs are marked
VM_SOFTDIRTY.

To account for this we use unfaulted anon VMAs, mapping one VMA in and
clearing soft-dirty, then another separate from the first which will be
marked soft-dirty which we then mremap() into place.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
---
 tools/testing/selftests/mm/soft-dirty.c | 51 ++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/soft-dirty.c b/tools/testing/selftests/mm/soft-dirty.c
index 4ee4db3750c1..bb29edb1e2a3 100644
--- a/tools/testing/selftests/mm/soft-dirty.c
+++ b/tools/testing/selftests/mm/soft-dirty.c
@@ -184,6 +184,54 @@ static void test_mprotect(int pagemap_fd, int pagesize, bool anon)
 		close(test_fd);
 }
 
+static void test_merge(int pagemap_fd, int pagesize)
+{
+	char *reserved, *map, *map2;
+
+	/* Reserve space. */
+	reserved = mmap(NULL, 4 * pagesize, PROT_NONE,
+			MAP_ANON | MAP_PRIVATE, -1, 0);
+	if (reserved == MAP_FAILED)
+		ksft_exit_fail_msg("mmap failed\n");
+	munmap(reserved, 4 * pagesize);
+
+	/* Map a page. */
+	map = mmap(&reserved[pagesize], pagesize, PROT_READ | PROT_WRITE,
+		   MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+	if (map == MAP_FAILED)
+		ksft_exit_fail_msg("mmap failed\n");
+
+	/* This will clear VM_SOFTDIRTY too. */
+	clear_softdirty();
+
+	/*
+	 * Now place a new mapping which will be marked VM_SOFTDIRTY. Away from
+	 * map.
+	 */
+	map2 = mmap(&reserved[3 * pagesize], pagesize, PROT_READ | PROT_WRITE,
+		    MAP_ANON | MAP_PRIVATE | MAP_FIXED, -1, 0);
+	if (map2 == MAP_FAILED)
+		ksft_exit_fail_msg("mmap failed\n");
+
+	/*
+	 * Now remap it immediately adjacent to map, if the merge correctly
+	 * propagates VM_SOFTDIRTY, we should then observe the VMA as a whole
+	 * being marked soft-dirty.
+	 */
+	map2 = mremap(map2, pagesize, pagesize, MREMAP_FIXED | MREMAP_MAYMOVE,
+		      &reserved[2 * pagesize]);
+	if (map2 == MAP_FAILED)
+		ksft_exit_fail_msg("mremap failed\n");
+	ksft_test_result(pagemap_is_softdirty(pagemap_fd, map) == 1,
+			 "Test %s-anon soft-dirty after merge 1st pg\n",
+			 __func__);
+	ksft_test_result(pagemap_is_softdirty(pagemap_fd, map2) == 1,
+			 "Test %s-anon soft-dirty after merge 2nd pg\n",
+			 __func__);
+
+	munmap(map, 2 * pagesize);
+}
+
 static void test_mprotect_anon(int pagemap_fd, int pagesize)
 {
 	test_mprotect(pagemap_fd, pagesize, true);
@@ -204,7 +252,7 @@ int main(int argc, char **argv)
 	if (!softdirty_supported())
 		ksft_exit_skip("soft-dirty is not support\n");
 
-	ksft_set_plan(15);
+	ksft_set_plan(17);
 	pagemap_fd = open(PAGEMAP_FILE_PATH, O_RDONLY);
 	if (pagemap_fd < 0)
 		ksft_exit_fail_msg("Failed to open %s\n", PAGEMAP_FILE_PATH);
@@ -216,6 +264,7 @@ int main(int argc, char **argv)
 	test_hugepage(pagemap_fd, pagesize);
 	test_mprotect_anon(pagemap_fd, pagesize);
 	test_mprotect_file(pagemap_fd, pagesize);
+	test_merge(pagemap_fd, pagesize);
 
 	close(pagemap_fd);
 
-- 
2.51.0