Re: [PATCH v3 3/7] selftest/mm: Fix ksm_funtional_test failures

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Jul 29, 2025 at 11:03:59AM +0530, Aboorva Devarajan wrote:
>From: Donet Tom <donettom@linux.ibm.com>
>
>This patch fixed 2 issues.
>
>1) After fork() in test_prctl_fork, the child process uses the file
>descriptors from the parent process to read ksm_stat and
>ksm_merging_pages. This results in incorrect values being read (parent
>process ksm_stat and ksm_merging_pages will be read in child), causing
>the test to fail.
>
>This patch calls init_global_file_handles() in the child process to
>ensure that the current process's file descriptors are used to read
>ksm_stat and ksm_merging_pages.
>
>2) All tests currently call ksm_merge to trigger page merging.
>To ensure the system remains in a consistent state for subsequent
>tests, it is better to call ksm_unmerge during the test cleanup phase.
>
>In the test_prctl_fork test, after a fork(), reading ksm_merging_pages
>in the child process returns a non-zero value because a previous test
>performed a merge, and the child's memory state is inherited from the
>parent.
>
>Although the child process calls ksm_unmerge, the ksm_merging_pages
>counter in the parent is reset to zero, while the child's counter
>remains unchanged. This discrepancy causes the test to fail.
>
>To avoid this issue, each test should call ksm_unmerge during cleanup
>to ensure the counter is reset and the system is in a clean state for
>subsequent tests.
>
>execv argument is an array of pointers to null-terminated strings.
>In this patch we also added NULL in the execv argument.
>
>Fixes: 6c47de3be3a0 ("selftest/mm: ksm_functional_tests: extend test case for ksm fork/exec")
>Co-developed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
>Signed-off-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
>Signed-off-by: Donet Tom <donettom@linux.ibm.com>
>---
> tools/testing/selftests/mm/ksm_functional_tests.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
>
>diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
>index d8bd1911dfc0..996dc6645570 100644
>--- a/tools/testing/selftests/mm/ksm_functional_tests.c
>+++ b/tools/testing/selftests/mm/ksm_functional_tests.c
>@@ -46,6 +46,8 @@ static int ksm_use_zero_pages_fd;
> static int pagemap_fd;
> static size_t pagesize;
> 
>+static void init_global_file_handles(void);
>+
> static bool range_maps_duplicates(char *addr, unsigned long size)
> {
> 	unsigned long offs_a, offs_b, pfn_a, pfn_b;
>@@ -274,6 +276,7 @@ static void test_unmerge(void)
> 	ksft_test_result(!range_maps_duplicates(map, size),
> 			 "Pages were unmerged\n");
> unmap:
>+	ksm_unmerge();

In __mmap_and_merge_range(), we call ksm_unmerge(). Why this one not help?

Not very familiar with ksm stuff. Would you mind giving more on how this fix
the failure you see?

> 	munmap(map, size);
> }
> 
>@@ -338,6 +341,7 @@ static void test_unmerge_zero_pages(void)
> 	ksft_test_result(!range_maps_duplicates(map, size),
> 			"KSM zero pages were unmerged\n");
> unmap:
>+	ksm_unmerge();
> 	munmap(map, size);
> }
> 
>@@ -366,6 +370,7 @@ static void test_unmerge_discarded(void)
> 	ksft_test_result(!range_maps_duplicates(map, size),
> 			 "Pages were unmerged\n");
> unmap:
>+	ksm_unmerge();
> 	munmap(map, size);
> }
> 
>@@ -452,6 +457,7 @@ static void test_unmerge_uffd_wp(void)
> close_uffd:
> 	close(uffd);
> unmap:
>+	ksm_unmerge();
> 	munmap(map, size);
> }
> #endif
>@@ -515,6 +521,7 @@ static int test_child_ksm(void)
> 	else if (map == MAP_MERGE_SKIP)
> 		return -3;
> 
>+	ksm_unmerge();
> 	munmap(map, size);
> 	return 0;
> }
>@@ -548,6 +555,7 @@ static void test_prctl_fork(void)
> 
> 	child_pid = fork();
> 	if (!child_pid) {
>+		init_global_file_handles();

Would this leave fd in parent as orphan?

> 		exit(test_child_ksm());
> 	} else if (child_pid < 0) {
> 		ksft_test_result_fail("fork() failed\n");
>@@ -595,7 +603,7 @@ static void test_prctl_fork_exec(void)
> 		return;
> 	} else if (child_pid == 0) {
> 		char *prg_name = "./ksm_functional_tests";
>-		char *argv_for_program[] = { prg_name, FORK_EXEC_CHILD_PRG_NAME };
>+		char *argv_for_program[] = { prg_name, FORK_EXEC_CHILD_PRG_NAME, NULL };
> 
> 		execv(prg_name, argv_for_program);
> 		return;
>@@ -644,6 +652,7 @@ static void test_prctl_unmerge(void)
> 	ksft_test_result(!range_maps_duplicates(map, size),
> 			 "Pages were unmerged\n");
> unmap:
>+	ksm_unmerge();
> 	munmap(map, size);
> }
> 
>@@ -677,6 +686,7 @@ static void test_prot_none(void)
> 	ksft_test_result(!range_maps_duplicates(map, size),
> 			 "Pages were unmerged\n");
> unmap:
>+	ksm_unmerge();
> 	munmap(map, size);
> }
> 
>-- 
>2.47.1
>

-- 
Wei Yang
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