Hello Christophe, thank you for the patch.

On Thu, 2020-02-06 at 08:42 +0000, Christophe Leroy wrote:
> Commit fa7b9a805c79 ("tools/selftest/vm: allow choosing mem size and
> page size in map_hugetlb") added the possibility to change the size
> of memory mapped for the test, but left the read and write test using
> the default value. This is unnoticed when mapping a length greater
> than the default one, but segfaults otherwise.
> 
> Fix read_bytes() and write_bytes() by giving them the real length.
> 
> Also fix the call to munmap().
> 
> Fixes: fa7b9a805c79 ("tools/selftest/vm: allow choosing mem size and page size in map_hugetlb")
> Cc: stable@vger.kernel.org
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> ---
>  tools/testing/selftests/vm/map_hugetlb.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/testing/selftests/vm/map_hugetlb.c b/tools/testing/selftests/vm/map_hugetlb.c
> index 5a2d7b8efc40..6af951900aa3 100644
> --- a/tools/testing/selftests/vm/map_hugetlb.c
> +++ b/tools/testing/selftests/vm/map_hugetlb.c
> @@ -45,20 +45,20 @@ static void check_bytes(char *addr)
>  	printf("First hex is %x\n", *((unsigned int *)addr));
>  }
>  
> -static void write_bytes(char *addr)
> +static void write_bytes(char *addr, size_t length)
>  {
>  	unsigned long i;
>  
> -	for (i = 0; i < LENGTH; i++)
> +	for (i = 0; i < length; i++)
>  		*(addr + i) = (char)i;
>  }
>  
> -static int read_bytes(char *addr)
> +static int read_bytes(char *addr, size_t length)
>  {
>  	unsigned long i;
>  
>  	check_bytes(addr);
> -	for (i = 0; i < LENGTH; i++)
> +	for (i = 0; i < length; i++)
>  		if (*(addr + i) != (char)i) {
>  			printf("Mismatch at %lu\n", i);
>  			return 1;
> @@ -96,11 +96,11 @@ int main(int argc, char **argv)
>  
>  	printf("Returned address is %p\n", addr);
>  	check_bytes(addr);
> -	write_bytes(addr);
> -	ret = read_bytes(addr);
> +	write_bytes(addr, length);
> +	ret = read_bytes(addr, length);
>  
>  	/* munmap() length of MAP_HUGETLB memory must be hugepage aligned */
> -	if (munmap(addr, LENGTH)) {
> +	if (munmap(addr, length)) {
>  		perror("munmap");
>  		exit(1);
>  	}

I agree with you, it's a needed fix.

FWIW:
Reviwed-by: Leonardo Bras <leonardo@linux.ibm.com>