[PATCH v11 3/3] ksm: add mremap selftests for ksm_rmap_walk

[xu.xin16]

From: xu xin <xu.xin16@xxxxxxxxxx>

The existing tools/testing/selftests/mm/rmap.c has already one testcase
for ksm_rmap_walk in TEST_F(migrate, ksm), which takes use of migration
of page from one NUMA node to another NUMA node. However, it just lacks
the scenario of mremapped VMAs.

We add the calling of mremap() and then trigger KSM to merge pages before
migrating, which is specifically to test an optimization which is
introduced by this patch ("ksm: Optimize rmap_walk_ksm by passing a
suitable address pgoff").

This test can reproduce the issue that Hugh points out at
https://lore.kernel.org/all/02e1b8df-d568-8cbb-b8f6-46d5476d9d75@xxxxxxxxxx/

Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
---
 tools/testing/selftests/mm/rmap.c | 81 +++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/tools/testing/selftests/mm/rmap.c b/tools/testing/selftests/mm/rmap.c
index 53f2058b0ef2..1c293ad3f8b8 100644
--- a/tools/testing/selftests/mm/rmap.c
+++ b/tools/testing/selftests/mm/rmap.c
@@ -430,4 +430,85 @@ TEST_F(migrate, ksm)
 	propagate_children(_metadata, data);
 }

+static bool range_maps_the_same_pfn(int pagemap_fd, void *region, int nr_pages)
+{
+	int i;
+	int retries = 0;
+	unsigned long first_pfn;
+
+retry:
+	if (retries > 10)
+		return false;
+
+	first_pfn = pagemap_get_pfn(pagemap_fd, region);
+	for (i = 0; i < nr_pages; i++) {
+		if (pagemap_get_pfn(pagemap_fd, region + i * getpagesize()) != first_pfn) {
+			/*
+			 * Retry up to 10 times at most in case of the low chance of page
+			 * compaction migrating the page while we check for pfn.
+			 */
+			retries++;
+			goto retry;
+		}
+	}
+
+	return true;
+}
+
+TEST_F(migrate, ksm_and_mremap)
+{
+	unsigned long old_pfn, new_pfn;
+	void *region, *mremap_region;
+	const int nr_pages = 16;
+	size_t mmap_size;
+	int pagemap_fd;
+
+	/* Skip if KSM is not available */
+	if (ksm_stop() < 0)
+		SKIP(return, "accessing \"/sys/kernel/mm/ksm/run\" failed");
+	if (ksm_get_full_scans() < 0)
+		SKIP(return, "accessing \"/sys/kernel/mm/ksm/full_scan\" failed");
+
+	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+	if (pagemap_fd < 0)
+		SKIP(return, "opening pagemap failed");
+
+	/* Allocate and populate twice the anon pages initially. */
+	mmap_size = 2 * nr_pages * getpagesize();
+	region = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
+		      MAP_PRIVATE | MAP_ANON, -1, 0);
+	ASSERT_NE(region, MAP_FAILED);
+	memset(region, 0x77, mmap_size);
+
+	/* mremap the second half over the first half, to stress rmap handling */
+	mmap_size /= 2;
+	mremap_region = mremap(region + mmap_size, mmap_size, mmap_size,
+			       MREMAP_MAYMOVE | MREMAP_FIXED, region);
+	ASSERT_EQ(mremap_region, region);
+
+	/* Merge all pages into a single KSM page. */
+	madvise(region, mmap_size, MADV_MERGEABLE);
+	ASSERT_EQ(ksm_start(), 0);
+
+	/* The whole range should map the same KSM page. */
+	old_pfn = pagemap_get_pfn(pagemap_fd, region);
+	if (old_pfn == -1ul)
+		SKIP(return, "Obtaining PFN failed");
+	ksm_start();
+	ASSERT_TRUE(range_maps_the_same_pfn(pagemap_fd, region, nr_pages));
+
+	/*
+	 * Migrate the KSM page; the whole range should map the new (migrated)
+	 * KSM page.
+	 */
+	ASSERT_EQ(try_to_move_page(region), 0);
+
+	new_pfn = pagemap_get_pfn(pagemap_fd, region);
+	if (new_pfn == -1ul)
+		SKIP(return, "Obtaining PFN failed");
+	ASSERT_NE(new_pfn, old_pfn);
+	ASSERT_TRUE(range_maps_the_same_pfn(pagemap_fd, region, nr_pages));
+}
+
+
 TEST_HARNESS_MAIN
-- 
2.25.1