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

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.