Re: [PATCH 2/6] selftests/mm: test a partial mremap() of hugetlb memory

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

On Fri, Oct 30, 2026 at 02:42:47PM +0000, Josef Bacik wrote:
> hugetlb-mremap only moves whole mappings, so it did not catch the
> reservation leak of a partial move.
>
> Map four private huge pages without touching them, move the second one,
> unmap everything, and check that the number of reserved huge pages is back
> where it was.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Looks sensible so:

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

> ---
>  tools/testing/selftests/mm/hugetlb-mremap.c | 52 +++++++++++++++++++++++++++--
>  1 file changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/hugetlb-mremap.c b/tools/testing/selftests/mm/hugetlb-mremap.c
> index 9b724af66e93..9f9d6905cd6d 100644
> --- a/tools/testing/selftests/mm/hugetlb-mremap.c
> +++ b/tools/testing/selftests/mm/hugetlb-mremap.c
> @@ -95,6 +95,51 @@ static void register_region_with_uffd(char *addr, size_t len)
>  	ksft_print_msg("Registered memory at address %p with userfaultfd\n", addr);
>  }
>
> +/*
> + * Move one page out of the middle of a private mapping that has not been
> + * touched, so that every page of it still holds its reservation.  Once all of
> + * it is unmapped, all of the reservations have to be back in the pool.
> + */
> +static void test_partial_move(unsigned long hugepage_size)
> +{
> +	const int flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB;
> +	const int prot = PROT_READ | PROT_WRITE;
> +	const unsigned long nr = 4;
> +	unsigned long resv = hugetlb_nr_resv_pages(hugepage_size);
> +	unsigned long resv_after;
> +	char *src, *dst, *addr;
> +
> +	/* The pages of the test before this one can still be in use. */
> +	if (!hugetlb_setup_default(resv + nr + 1)) {
> +		ksft_test_result_skip("Partial move: not enough free huge pages\n");
> +		return;
> +	}
> +
> +	src = mmap(NULL, nr * hugepage_size, prot, flags, -1, 0);
> +	if (src == MAP_FAILED)
> +		ksft_exit_fail_msg("mmap src: %s\n", strerror(errno));
> +
> +	/* The destination has to be hugetlb-backed, mremap() replaces it. */
> +	dst = mmap(NULL, hugepage_size, prot, flags, -1, 0);
> +	if (dst == MAP_FAILED)
> +		ksft_exit_fail_msg("mmap dst: %s\n", strerror(errno));
> +
> +	addr = mremap(src + hugepage_size, hugepage_size, hugepage_size,
> +		      MREMAP_MAYMOVE | MREMAP_FIXED, dst);
> +	if (addr == MAP_FAILED)
> +		ksft_exit_fail_msg("partial mremap: %s\n", strerror(errno));
> +
> +	munmap(addr, hugepage_size);
> +	munmap(src, nr * hugepage_size);
> +
> +	resv_after = hugetlb_nr_resv_pages(hugepage_size);
> +	if (resv_after != resv)
> +		ksft_print_msg("Reserved huge pages: %lu before, %lu after\n",
> +			       resv, resv_after);
> +	ksft_test_result(resv_after == resv,
> +			 "Partial move returns all reservations\n");
> +}
> +
>  int main(int argc, char *argv[])
>  {
>  	unsigned long hugepage_size;
> @@ -102,7 +147,7 @@ int main(int argc, char *argv[])
>  	size_t length = 0;
>
>  	ksft_print_header();
> -	ksft_set_plan(1);
> +	ksft_set_plan(2);
>
>  	if (argc >= 2 && !strcmp(argv[1], "-h"))
>  		ksft_exit_fail_msg("Usage: %s [length_in_MB]\n", argv[0]);
> @@ -170,5 +215,8 @@ int main(int argc, char *argv[])
>  	close(fd);
>
>  	ksft_test_result(!ret, "Read same data\n");
> -	ksft_exit(!ret);
> +
> +	test_partial_move(hugepage_size);
> +
> +	ksft_finished();
>  }
>
> --
> 2.55.0
>

--
Cheers, Lorenzo