Re: [PATCH v2] selftests/mm: khugepaged: initialize file contents via mmap

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/28/26 11:43, Vineet Agarwal wrote:
> file_setup_area() currently allocates anonymous memory, fills it,
> and writes it into the backing file used for collapse testing.
> 
> Instead of copying data through write(), map the file directly with
> MAP_SHARED after resizing it with ftruncate(), initialize the mapped
> area in place, and sync it with msync().
> 
> This simplifies the setup path and avoids the need for explicit
> partial write handling.
> 
> Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
> ---
>  tools/testing/selftests/mm/khugepaged.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 3fe7ef04ac62..f6e171e1da4f 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -369,7 +369,6 @@ static void *file_setup_area(int nr_hpages)
>  	int fd;
>  	void *p;
>  	unsigned long size;
> -
>  	unlink(finfo.path);  /* Cleanup from previous failed tests */
>  	printf("Creating %s for collapse%s...", finfo.path,
>  	       finfo.type == VMA_SHMEM ? " (tmpfs)" : "");
> @@ -381,11 +380,29 @@ static void *file_setup_area(int nr_hpages)
>  	}
>  
>  	size = nr_hpages * hpage_pmd_size;
> -	p = alloc_mapping(nr_hpages);
> +	if (ftruncate(fd, size)) {
> +		perror("ftruncate()");
> +		close(fd);
> +		exit(EXIT_FAILURE);
> +	}

Why is that required? We write all pages.

> +	p = mmap(BASE_ADDR, size, PROT_READ | PROT_WRITE,
> +		MAP_SHARED, fd, 0);
> +	if (p == MAP_FAILED || p != BASE_ADDR) {
> +		perror("mmap()");
> +		close(fd);
> +		exit(EXIT_FAILURE);
> +	}
>  	fill_memory(p, 0, size);
> -	write(fd, p, size);
> -	close(fd);
> +
> +	if (msync(p, size, MS_SYNC)) {
> +		perror("msync()");
> +		munmap(p, size);
> +		close(fd);
> +		exit(EXIT_FAILURE);

Why is that required?

-- 
Cheers,

David