Re: [PATCH] selftests/mm: khugepaged: handle partial write in file_setup_area()

David Hildenbrand (Arm)
2026-04-28T08:04:39Z

On 4/28/26 08:23, Vineet Agarwal wrote:
> file_setup_area() writes initialized test data into the backing
> file used for collapse testing, but it ignores the return value
> from write().
>
> If write() fails or completes only partially, the test continues
> with incomplete file contents, which can lead to incorrect test
> results and make failures harder to diagnose.
>
> Handle partial writes by retrying until all data is written and
> abort the test if write() fails.
>
> Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
> ---
> tools/testing/selftests/mm/khugepaged.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/khugepaged.c b/tools/testing/selftests/mm/khugepaged.c
> index 3fe7ef04ac62..c13f06758750 100644
> --- a/tools/testing/selftests/mm/khugepaged.c
> +++ b/tools/testing/selftests/mm/khugepaged.c
> @@ -369,7 +369,9 @@ static void *file_setup_area(int nr_hpages)
> int fd;
> void *p;
> unsigned long size;
> -
> + size_t remaining;
> + ssize_t ret;
> + char *buf;
> unlink(finfo.path); /* Cleanup from previous failed tests */
> printf("Creating %s for collapse%s...", finfo.path,
> finfo.type == VMA_SHMEM ? " (tmpfs)" : "");
> @@ -383,7 +385,19 @@ static void *file_setup_area(int nr_hpages)
> size = nr_hpages * hpage_pmd_size;
> p = alloc_mapping(nr_hpages);
> fill_memory(p, 0, size);
> - write(fd, p, size);
> + remaining = size;
> + buf = p;
> +
> + while (remaining > 0) {
> + ret = write(fd, buf, remaining);
> + if (ret <= 0) {
> + close(fd);
> + munmap(p, size);
> + ksft_exit_fail_msg("write() failed while preparing test file\n");
> + }
> + buf += ret;
> + remaining -= ret;
> + }

Why are we even using write() instead of just mapping the fd and writing to that
area?

diff --git a/tools/testing/selftests/mm/khugepaged.c
b/tools/testing/selftests/mm/khugepaged.c
index 3fe7ef04ac62..a77f753ccf38 100644
--- a/tools/testing/selftests/mm/khugepaged.c
+++ b/tools/testing/selftests/mm/khugepaged.c
@@ -381,9 +381,10 @@ static void *file_setup_area(int nr_hpages)
}

size = nr_hpages * hpage_pmd_size;
- p = alloc_mapping(nr_hpages);
+ p = mmap(BASE_ADDR, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+ if (p != BASE_ADDR)
+ ksft_exit_fail_msg("mmap() failed while preparing test file\n");
fill_memory(p, 0, size);
- write(fd, p, size);
close(fd);
munmap(p, size);
success("OK");

--
Cheers,

David