Re: [PATCH v5 13/19] selftests/mm: cover a shared-source collapse write race

[Kiryl Shutsemau]

On Tue, Sep 08, 2026 at 01:50:59PM +0100, Kiryl Shutsemau wrote:
> +	close(sync[1]);
> +	if (read(sync[0], &go, 1) != 1)
> +		ksft_exit_fail_msg("child never reached the collapse\n");
> +
> +	/*
> +	 * Unshare one page at a time: a burst would break CoW on the whole
> +	 * range before the collapse starts, leaving nothing shared to collapse.
> +	 */
> +	i = 0;
> +	for (;;) {
> +		if (i < n)
> +			ip[i * stride] = i + 0xbeef0000;
> +		i++;
> +		usleep(10 * 1000);
> +		if (waitpid(child, &wstatus, WNOHANG))
> +			break;
> +	}

Sashiko flagged two issues here:

 - sync[0] never closed. It is fd leak;

 - wstatus is garbage if waitpid() fails, returning -1;

Here's a fixup:

diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
index 335f946eca61..c0a07d75a306 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -1228,6 +1228,7 @@ static void collapse_fork_cow_race(struct collapse_context *c, struct mem_ops *o
 	close(sync[1]);
 	if (read(sync[0], &go, 1) != 1)
 		ksft_exit_fail_msg("child never reached the collapse\n");
+	close(sync[0]);
 
 	/*
 	* Unshare one page at a time: a burst would break CoW on the whole
@@ -1235,12 +1236,17 @@ static void collapse_fork_cow_race(struct collapse_context *c, struct mem_ops *o
 	*/
 	i = 0;
 	for (;;) {
+		pid_t ret;
+
 		if (i < n)
 			ip[i * stride] = i + 0xbeef0000;
 		i++;
 		usleep(10 * 1000);
-		if (waitpid(child, &wstatus, WNOHANG))
+		ret = waitpid(child, &wstatus, WNOHANG);
+		if (ret == child)
 			break;
+		if (ret < 0)
+			ksft_exit_fail_perror("waitpid()");
 	}
 
 	/* Finish whatever the paced sweep did not reach */
-- 
  Kiryl Shutsemau / Kirill A. Shutemov