linux-kernel - Re: [PATCH v9 5/5] selftest: add a testcase of ksm zero pages

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Message-ID: <84c00f9d-0b83-e4a2-de6f-617cf23b6444@redhat.com>
Date:   Wed, 24 May 2023 11:11:40 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Yang Yang <yang.yang29@....com.cn>, akpm@...ux-foundation.org
Cc:     imbrenda@...ux.ibm.com, jiang.xuexin@....com.cn,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        ran.xiaokai@....com.cn, xu.xin.sc@...il.com, xu.xin16@....com.cn
Subject: Re: [PATCH v9 5/5] selftest: add a testcase of ksm zero pages

[...]

> +static void test_unmerge_zero_pages(void)
> +{
> +	const unsigned int size = 2 * MiB;
> +	char *map;
> +	unsigned long pages_expected;
> +
> +	ksft_print_msg("[RUN] %s\n", __func__);
> +
> +	if (ksm_zero_pages_fd < 0) {
> +		ksft_test_result_skip("open(\"/sys/kernel/mm/ksm/ksm_zero_pages\") failed\n");
> +		return;
> +	}
> +	if (ksm_use_zero_pages_fd < 0) {
> +		ksft_test_result_skip("open \"/sys/kernel/mm/ksm/use_zero_pages\" failed\n");
> +		return;
> +	}
> +	if (write(ksm_use_zero_pages_fd, "1", 1) != 1) {
> +		ksft_test_result_skip("write \"/sys/kernel/mm/ksm/use_zero_pages\" failed\n");
> +		return;
> +	}

I realize that this test will fail if there is any other process in the system that has KSM
enabled with a suitable page filled with zeroes ... maybe instead of checking the global
KSM zeropages, check the per-mm ones instead? That should be better.

> +
> +	/* Mmap zero pages */

Maybe better:

/* Let KSM deduplicate zero pages. */

> +	map = mmap_and_merge_range(0x00, size, false);
> +	if (map == MAP_FAILED)
> +		return;
> +
> +	/* Check if ksm_zero_pages can be update correctly after merging */

s/can be updated/is updated/

> +	pages_expected = size / pagesize;
> +	if (pages_expected != get_ksm_zero_pages()) {
> +		ksft_test_result_fail("'ksm_zero_pages' updated after merging\n");
> +		goto unmap;
> +	}
> +
> +	/* try to unmerge half of the region */
> +	if (madvise(map, size / 2, MADV_UNMERGEABLE)) {
> +		ksft_test_result_fail("MADV_UNMERGEABLE failed\n");
> +		goto unmap;
> +	}
> +
> +	/* Check if ksm_zero_pages can be update correctly after unmerging */
> +	pages_expected /= 2;
> +	if (pages_expected != get_ksm_zero_pages()) {
> +		ksft_test_result_fail("'ksm_zero_pages' updated after unmerging\n");
> +		goto unmap;
> +	}

You could do something like the following on top (I recall you had some kind of COW
tests previously, this should be a simplified version of it):

diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
index ea060c683c80..160675a4e3d2 100644
--- a/tools/testing/selftests/mm/ksm_functional_tests.c
+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -182,6 +182,7 @@ static void test_unmerge(void)
  static void test_unmerge_zero_pages(void)
  {
  	const unsigned int size = 2 * MiB;
+	unsigned int offs;
  	char *map;
  	unsigned long pages_expected;
  
@@ -225,8 +226,18 @@ static void test_unmerge_zero_pages(void)
  		goto unmap;
  	}
  
+	/* Trigger unmerging of the other half by writing to the pages. */
+	for (offs = size / 2; offs < size; offs += pagesize)
+		*((unsigned int *)&map[offs]) = offs;
+
+	/* We should have no zeropages remaining. */
+	if (get_ksm_zero_pages()) {
+		ksft_test_result_fail("'ksm_zero_pages' updated after write fault\n");
+		goto unmap;
+	}
+
  	/* Check if ksm zero pages are really unmerged */
-	ksft_test_result(!range_maps_duplicates(map, size / 2),
+	ksft_test_result(!range_maps_duplicates(map, size),
  				"KSM zero pages were unmerged\n");
  unmap:
  	munmap(map, size);
-- 
2.40.1


You could also test manual discard (MADV_DONTNEED) and munmap(). Just a thought about which
tests to add eventually later.

-- 
Thanks,

David / dhildenb