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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Fri, 8 Apr 2022 18:21:55 -0700
Message-ID: <CAJD7tkb6VJt=pfqnW11r6S7A0r2Vh85a3YZaVso-qyiCM06nDQ@mail.gmail.com>
Subject: Re: [PATCH v3 2/4] selftests: cgroup: return the errno of write() in
 cg_write() on failure
To:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <roman.gushchin@linux.dev>
Cc:     David Rientjes <rientjes@google.com>, Tejun Heo <tj@kernel.org>,
        Zefan Li <lizefan.x@bytedance.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Shuah Khan <shuah@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Wei Xu <weixugc@google.com>, Greg Thelen <gthelen@google.com>,
        Chen Wandun <chenwandun@huawei.com>,
        Vaibhav Jain <vaibhav@linux.ibm.com>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, cgroups@vger.kernel.org,
        linux-doc@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, linux-kselftest@vger.kernel.org
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On Thu, Apr 7, 2022 at 9:57 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> Currently, cg_write() returns 0 on success and -1 on failure. Modify it
> to return the errno of write() syscall when write() fails.
>
> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> ---
>  tools/testing/selftests/cgroup/cgroup_util.c | 32 +++++++++++---------
>  1 file changed, 17 insertions(+), 15 deletions(-)
>
> diff --git a/tools/testing/selftests/cgroup/cgroup_util.c b/tools/testing/selftests/cgroup/cgroup_util.c
> index dbaa7aabbb4a..3b6bb09985fa 100644
> --- a/tools/testing/selftests/cgroup/cgroup_util.c
> +++ b/tools/testing/selftests/cgroup/cgroup_util.c
> @@ -38,23 +38,23 @@ static ssize_t read_text(const char *path, char *buf, size_t max_len)
>         return len;
>  }
>
> -static ssize_t write_text(const char *path, char *buf, ssize_t len)
> +/*
> + * Returns:
> + *     success -> 0
> + *     open() failure -> -1
> + *     write() failure -> errno
> + */
> +static int write_text(const char *path, char *buf, ssize_t len)
>  {
> -       int fd;
> +       int fd, ret;
>
>         fd = open(path, O_WRONLY | O_APPEND);
>         if (fd < 0)
>                 return fd;
>
> -       len = write(fd, buf, len);
> -       if (len < 0) {
> -               close(fd);
> -               return len;
> -       }
> -
> +       ret = write(fd, buf, len) < 0 ? errno : 0;
>         close(fd);
> -
> -       return len;
> +       return ret;
>  }
>
>  char *cg_name(const char *root, const char *name)
> @@ -177,17 +177,19 @@ long cg_read_lc(const char *cgroup, const char *control)
>         return cnt;
>  }
>
> +/*
> + * Returns:
> + *     success -> 0
> + *     open() failure -> -1
> + *     write() failure -> errno
> + */
>  int cg_write(const char *cgroup, const char *control, char *buf)
>  {
>         char path[PATH_MAX];
>         ssize_t len = strlen(buf);
>
>         snprintf(path, sizeof(path), "%s/%s", cgroup, control);
> -
> -       if (write_text(path, buf, len) == len)
> -               return 0;
> -
> -       return -1;
> +       return write_text(path, buf, len);
>  }

I have changed this in v4 to a cleaner implementation that either
returns 0 on success or -errno on failure. I also made sure to check
that the full buffer was being written, and updated cg_read() as well
for the interface to be consistent.

Will send out once the discussion on patch 1 in v3 reaches a consensus.

>
>  int cg_find_unified_root(char *root, size_t len)
> --
> 2.35.1.1178.g4f1659d476-goog
>