Re: [PATCH v3 3/5] selftests/mm: move write_file helper to vm_util

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Tue, Mar 17, 2026 at 05:44:17PM +0800, Chunyu Hu wrote:
> thp_settings provides write_file() helper for safely writing to a file and
> exit when write failure happens. It's a very low level helper and many sub
> tests need such a helper, not only thp tests.
>
> split_huge_page_test also defines a write_file locally. The two have minior
> differences in return type and used exit api. And there would be conflicts
> if split_huge_page_test wanted to include thp_settings.h because of
> different prototype, making it less convenient.
>
> It's possisble to merge the two, although some tests don't use the
> kselftest infrastrucutre for testing. It would also work when using the
> ksft_exit_msg() to exit in my test, as the counters are all zero. Output
> will be like:
>
>   TAP version 13
>   1..62
>   Bail out! /proc/sys/vm/drop_caches1 open failed: No such file or directory
>   # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
>
> So here we just keep the version in split_huge_page_test, and move it into
> the vm_util. This makes it easy to maitain and user could just include one
> vm_util.h when they don't need thp setting helpers. Keep the prototype of
> void return as the function will exit on any error, return value is not
> necessary, and will simply the callers like write_num() and write_string().
>
> Suggested-by: Mike Rapoport <rppt@kernel.org>
> Signed-off-by: Chunyu Hu <chuhu@redhat.com>

LGTM and broadly a good idea, I do share David's concern about what if tests
that don't use the usual framework have an issue, but really that's an issue
with _those tests_ more so than this.

Anyway, nice cleanup so:

Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>

> ---
> Changes in v3:
>   - make the write_file void return
> Changes in v2:
>   new patch from v2
> ---
>  .../selftests/mm/split_huge_page_test.c       | 15 --------
>  tools/testing/selftests/mm/thp_settings.c     | 35 ++-----------------
>  tools/testing/selftests/mm/thp_settings.h     |  1 -
>  tools/testing/selftests/mm/vm_util.c          | 15 ++++++++
>  tools/testing/selftests/mm/vm_util.h          |  2 ++
>  5 files changed, 20 insertions(+), 48 deletions(-)
>
> diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> index e0167111bdd1..93f205327b84 100644
> --- a/tools/testing/selftests/mm/split_huge_page_test.c
> +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> @@ -255,21 +255,6 @@ static int check_after_split_folio_orders(char *vaddr_start, size_t len,
>  	return status;
>  }
>
> -static void write_file(const char *path, const char *buf, size_t buflen)
> -{
> -	int fd;
> -	ssize_t numwritten;
> -
> -	fd = open(path, O_WRONLY);
> -	if (fd == -1)
> -		ksft_exit_fail_msg("%s open failed: %s\n", path, strerror(errno));
> -
> -	numwritten = write(fd, buf, buflen - 1);
> -	close(fd);
> -	if (numwritten < 1)
> -		ksft_exit_fail_msg("Write failed\n");
> -}
> -
>  static void write_debugfs(const char *fmt, ...)
>  {
>  	char input[INPUT_MAX];
> diff --git a/tools/testing/selftests/mm/thp_settings.c b/tools/testing/selftests/mm/thp_settings.c
> index 574bd0f8ae48..e748ebfb3d4e 100644
> --- a/tools/testing/selftests/mm/thp_settings.c
> +++ b/tools/testing/selftests/mm/thp_settings.c
> @@ -6,6 +6,7 @@
>  #include <string.h>
>  #include <unistd.h>
>
> +#include "vm_util.h"
>  #include "thp_settings.h"
>
>  #define THP_SYSFS "/sys/kernel/mm/transparent_hugepage/"
> @@ -64,29 +65,6 @@ int read_file(const char *path, char *buf, size_t buflen)
>  	return (unsigned int) numread;
>  }
>
> -int write_file(const char *path, const char *buf, size_t buflen)
> -{
> -	int fd;
> -	ssize_t numwritten;
> -
> -	fd = open(path, O_WRONLY);
> -	if (fd == -1) {
> -		printf("open(%s)\n", path);
> -		exit(EXIT_FAILURE);
> -		return 0;
> -	}
> -
> -	numwritten = write(fd, buf, buflen - 1);
> -	close(fd);
> -	if (numwritten < 1) {
> -		printf("write(%s)\n", buf);
> -		exit(EXIT_FAILURE);
> -		return 0;
> -	}
> -
> -	return (unsigned int) numwritten;
> -}
> -
>  unsigned long read_num(const char *path)
>  {
>  	char buf[21];
> @@ -104,10 +82,7 @@ void write_num(const char *path, unsigned long num)
>  	char buf[21];
>
>  	sprintf(buf, "%ld", num);
> -	if (!write_file(path, buf, strlen(buf) + 1)) {
> -		perror(path);
> -		exit(EXIT_FAILURE);
> -	}
> +	write_file(path, buf, strlen(buf) + 1);
>  }
>
>  int thp_read_string(const char *name, const char * const strings[])
> @@ -165,11 +140,7 @@ void thp_write_string(const char *name, const char *val)
>  		printf("%s: Pathname is too long\n", __func__);
>  		exit(EXIT_FAILURE);
>  	}
> -
> -	if (!write_file(path, val, strlen(val) + 1)) {
> -		perror(path);
> -		exit(EXIT_FAILURE);
> -	}
> +	write_file(path, val, strlen(val) + 1);
>  }
>
>  unsigned long thp_read_num(const char *name)
> diff --git a/tools/testing/selftests/mm/thp_settings.h b/tools/testing/selftests/mm/thp_settings.h
> index 76eeb712e5f1..7748a9009191 100644
> --- a/tools/testing/selftests/mm/thp_settings.h
> +++ b/tools/testing/selftests/mm/thp_settings.h
> @@ -63,7 +63,6 @@ struct thp_settings {
>  };
>
>  int read_file(const char *path, char *buf, size_t buflen);
> -int write_file(const char *path, const char *buf, size_t buflen);
>  unsigned long read_num(const char *path);
>  void write_num(const char *path, unsigned long num);
>
> diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
> index a6d4ff7dfdc0..ad96d19d1b85 100644
> --- a/tools/testing/selftests/mm/vm_util.c
> +++ b/tools/testing/selftests/mm/vm_util.c
> @@ -764,3 +764,18 @@ int unpoison_memory(unsigned long pfn)
>
>  	return ret > 0 ? 0 : -errno;
>  }
> +
> +void write_file(const char *path, const char *buf, size_t buflen)
> +{
> +	int fd;
> +	ssize_t numwritten;
> +
> +	fd = open(path, O_WRONLY);
> +	if (fd == -1)
> +		ksft_exit_fail_msg("%s open failed: %s\n", path, strerror(errno));
> +
> +	numwritten = write(fd, buf, buflen - 1);
> +	close(fd);
> +	if (numwritten < 1)
> +		ksft_exit_fail_msg("Write failed\n");
> +}
> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> index e9c4e24769c1..1a07305ceff4 100644
> --- a/tools/testing/selftests/mm/vm_util.h
> +++ b/tools/testing/selftests/mm/vm_util.h
> @@ -166,3 +166,5 @@ int unpoison_memory(unsigned long pfn);
>
>  #define PAGEMAP_PRESENT(ent)	(((ent) & (1ull << 63)) != 0)
>  #define PAGEMAP_PFN(ent)	((ent) & ((1ull << 55) - 1))
> +
> +void write_file(const char *path, const char *buf, size_t buflen);
> --
> 2.53.0
>