Re: [PATCH v5 16/19] selftests/mm: add khugepaged race harness

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, Sep 08, 2026 at 01:51:02PM +0100, Kiryl Shutsemau wrote:
> +static void *mremapper_fn(void *arg)
> +{
> +	unsigned int seed = (unsigned long)arg;
> +
> +	while (!stop) {
> +		void *p;
> +
> +		p = mremap(mremap_area, hpage_pmd_size, hpage_pmd_size,
> +			   MREMAP_MAYMOVE | MREMAP_FIXED, mremap_scratch);
> +		if (p == MAP_FAILED)
> +			ksft_exit_fail_perror("mremap() away");
> +		for (int i = 0; i < 8; i++)
> +			mremap_scratch[(rand_r(&seed) %
> +				(hpage_pmd_size / page_size)) * page_size] = 1;
> +		p = mremap(mremap_scratch, hpage_pmd_size, hpage_pmd_size,
> +			   MREMAP_MAYMOVE | MREMAP_FIXED, mremap_area);
> +		if (p == MAP_FAILED)
> +			ksft_exit_fail_perror("mremap() back");
> +		usleep(rand_r(&seed) % 2000);
> +	}
> +	return NULL;
> +}

Sashiko pointed out that mremap() freed scratch area and it has to be
re-reserved for next cycle.

Here's fixup:

diff --git a/tools/testing/selftests/mm/khugepaged_race.c b/tools/testing/selftests/mm/khugepaged_race.c
index c3478b5123e2..ccfe655b3ddd 100644
--- a/tools/testing/selftests/mm/khugepaged_race.c
+++ b/tools/testing/selftests/mm/khugepaged_race.c
@@ -209,6 +209,11 @@ static void *mremapper_fn(void *arg)
 			  MREMAP_MAYMOVE | MREMAP_FIXED, mremap_area);
 		if (p == MAP_FAILED)
 			ksft_exit_fail_perror("mremap() back");
+		/* The move back unmapped the scratch address: claim it again */
+		if (mmap(mremap_scratch, hpage_pmd_size, PROT_NONE,
+			MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED_NOREPLACE,
+			-1, 0) != (void *)mremap_scratch)
+			ksft_exit_fail_perror("mmap() mremap scratch");
 		usleep(rand_r(&seed) % 2000);
 	}
 	return NULL;
-- 
  Kiryl Shutsemau / Kirill A. Shutemov