linux-kernel - Re: [PATCH] mm: avoid implicit type conversion

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Message-ID: <20250107040631.GN1977892@ZenIV>
Date: Tue, 7 Jan 2025 04:06:31 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Kenny Cheng <chao.shun.cheng.tw@...il.com>
Cc: hannes@...xchg.org, muchun.song@...ux.dev, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	c.s.cheng@...ltek.com
Subject: Re: [PATCH] mm: avoid implicit type conversion

On Tue, Jan 07, 2025 at 11:51:41AM +0800, Kenny Cheng wrote:
> The function 'task_in_memcg_oom' returns a 'struct mem_cgroup *' type.
> If the compiler does not inline this function, a compile error occurs,
> as shown below:
> 
> ./include/linux/memcontrol.h:961:9: error: incompatible pointer to 
> integer conversion returning 'struct mem_cgroup *' from a function with
> result type 'unsigned char' [-Wint-conversion]

What does inlining have to do with anything?  And where has unsigned
char come from in that?
 
> This patch avoids the implicit type conversion by ensuring the return
> type is correct.
> 
> Signed-off-by: Kenny Cheng <chao.shun.cheng.tw@...il.com>
> ---
>  include/linux/memcontrol.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 5502aa8e138e..47acf1e4f5a7 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1826,7 +1826,7 @@ bool mem_cgroup_oom_synchronize(bool wait);
>  
>  static inline bool task_in_memcg_oom(struct task_struct *p)
>  {
> -	return p->memcg_in_oom;
> +	return !!p->memcg_in_oom;
>  }

That makes no sense.  Do you have bool (or _Bool) defined or typedefed
to unsigned char somewhere?  If so, that's the bug that needs to be
fixed.