Re: [PATCH v2] selftests/mm: ksm-functional-tests: fix partial write handling

Mike Rapoport
2026-05-03T08:48:43Z

On Sun, May 03, 2026 at 11:30:30AM +0530, Vineet Agarwal wrote:
> Update write() checks to properly detect and handle partial writes.
>
> Previously, partial writes (ret > 0 && ret != len) would return
> -errno, but write() does not set errno in this case. This could
> result in returning 0 and incorrectly signaling success.
>
> Fix this by:
> - returning -errno only on actual failures (ret < 0)
> - returning -EIO when a partial write is detected
>
> This ensures partial writes are treated as errors and prevents
> false success reporting in tests.
>
> Signed-off-by: Vineet Agarwal <agarwal.vineet2006@gmail.com>
>
> Changes in v2:
> - Fix incorrect use of -errno on partial writes
> - Return -EIO when write() completes partially

Please start a new thread for a new version of the patch.

> ---
> tools/testing/selftests/mm/ksm_functional_tests.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/ksm_functional_tests.c b/tools/testing/selftests/mm/ksm_functional_tests.c
> index 9a8c852acd68..c80254cda926 100644
> --- a/tools/testing/selftests/mm/ksm_functional_tests.c
> +++ b/tools/testing/selftests/mm/ksm_functional_tests.c
> @@ -507,9 +507,13 @@ static int start_ksmd_and_set_frequency(char *pages_to_scan, char *sleep_ms)
> if (write(ksm_fd, "1", 1) != 1)
> return -errno;
>
> - ret = write(pages_to_scan_fd, pages_to_scan, strlen(pages_to_scan));
> - if (ret < 0 || ret != strlen(pages_to_scan))
> + ssize_t len = strlen(pages_to_scan);

The v2 diff should be against the base, not against v1.

> +
> + ret = write(pages_to_scan_fd, pages_to_scan, len);
> + if (ret < 0)
> return -errno;
> + if (ret != len)
> + return -EIO;

Just return -1 if write() != strlen(), the actual error code does not
matter anyway.

> ret = write(sleep_millisecs_fd, sleep_ms, strlen(sleep_ms));
> if (ret < 0 || ret != strlen(sleep_ms))
> @@ -531,8 +535,10 @@ static int stop_ksmd_and_restore_frequency(void)
> return -errno;
>
> ret = write(pages_to_scan_fd, "100", 3);
> - if (ret < 0 || ret != 3)
> + if (ret < 0)
> return -errno;
> + if (ret != 3)
> + return -EIO;
>
> ret = write(sleep_millisecs_fd, "20", 2);
> if (ret < 0 || ret != 2)
> --
> 2.54.0
>

--
Sincerely yours,
Mike.