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