Re: [PATCH 5/6] selftests/mm: test mremap() with an old size of 0

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Fri, Oct 30, 2026 at 02:42:50PM +0000, Josef Bacik wrote:
> mremap() with an old size of 0 duplicates a shared mapping, and mremap_test
> has no case for it.
>
> Duplicate a shared anonymous mapping, once from its start and once from the
> middle. Check that the source and the duplicate are mapped by one VMA each
> and that they show the same pages.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Looks sensible so:

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  tools/testing/selftests/mm/mremap_test.c | 52 +++++++++++++++++++++++++++++++-
>  1 file changed, 51 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/mm/mremap_test.c b/tools/testing/selftests/mm/mremap_test.c
> index 31b484517bda..92960fff9537 100644
> --- a/tools/testing/selftests/mm/mremap_test.c
> +++ b/tools/testing/selftests/mm/mremap_test.c
> @@ -603,6 +603,54 @@ static void mremap_shrink_multiple_vmas(unsigned long page_size,
>  				      inplace ? " [inplace]" : "");
>  }
>
> +/*
> + * An old size of 0 duplicates a shared mapping: the pages show up at a second
> + * address, and the mapping they were taken from stays as it is.
> + */
> +static void mremap_duplicate_shared(FILE *maps_fp, unsigned long page_size,
> +				    unsigned long offset)
> +{
> +	char *test_name = "mremap duplicate shared mapping";
> +	const size_t size = 3 * page_size;
> +	const size_t dup_size = size - offset;
> +	char *dup = MAP_FAILED;
> +	bool success = false;
> +	char *ptr;
> +
> +	ptr = mmap(NULL, size, PROT_READ | PROT_WRITE,
> +		   MAP_SHARED | MAP_ANON, -1, 0);
> +	if (ptr == MAP_FAILED) {
> +		perror("mmap");
> +		goto out;
> +	}
> +	memset(ptr, 'a', size);
> +
> +	dup = mremap(ptr + offset, 0, dup_size, MREMAP_MAYMOVE);
> +	if (dup == MAP_FAILED) {
> +		perror("mremap");
> +		goto out_unmap;
> +	}
> +
> +	/* Both are mapped, each by a single VMA. */
> +	if (!is_ptr_mapped(maps_fp, ptr, size) ||
> +	    !is_ptr_mapped(maps_fp, dup, dup_size))
> +		goto out_unmap;
> +
> +	/* They show the same pages. */
> +	ptr[offset] = 'b';
> +	dup[dup_size - 1] = 'c';
> +	success = dup[0] == 'b' && ptr[size - 1] == 'c' && ptr[0] != 'c';
> +
> +out_unmap:
> +	if (dup != MAP_FAILED && munmap(dup, dup_size))
> +		perror("munmap dup");
> +	if (munmap(ptr, size))
> +		perror("munmap src");
> +out:
> +	ksft_test_result(success, "%s%s\n", test_name,
> +			 offset ? " [from the middle]" : "");
> +}
> +
>  static void mremap_move_multiple_vmas_split(unsigned int pattern_seed,
>  					    unsigned long page_size,
>  					    bool dont_unmap)
> @@ -1248,7 +1296,7 @@ int main(int argc, char **argv)
>  	char *rand_addr;
>  	size_t rand_size;
>  	int num_expand_tests = 2;
> -	int num_misc_tests = 9;
> +	int num_misc_tests = 11;
>  	struct test test_cases[MAX_TEST] = {};
>  	struct test perf_test_cases[MAX_PERF_TEST];
>  	int page_size;
> @@ -1385,6 +1433,8 @@ int main(int argc, char **argv)
>  	mremap_move_multiple_vmas_split(pattern_seed, page_size, /* dontunmap= */ false);
>  	mremap_move_multiple_vmas_split(pattern_seed, page_size, /* dontunmap= */ true);
>  	mremap_move_multi_invalid_vmas(maps_fp, page_size);
> +	mremap_duplicate_shared(maps_fp, page_size, 0);
> +	mremap_duplicate_shared(maps_fp, page_size, page_size);
>
>  	fclose(maps_fp);
>
>
> --
> 2.55.0
>

--
Cheers, Lorenzo