Re: linux-next: build warning after merge of the vfs-brauner tree

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Christian Brauner <brauner@kernel.org>,
	Tong Tiangen <tongtiangen@huawei.com>,
	 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	 Linux Next Mailing List <linux-next@vger.kernel.org>
Subject: Re: linux-next: build warning after merge of the vfs-brauner tree
Date: Tue, 5 Mar 2024 20:47:40 -0800	[thread overview]
Message-ID: <CAHk-=wiJLeR6tPcEmiRndhVtrPex0WavrFZ0UZHGjABVc=Tq1w@mail.gmail.com> (raw)
In-Reply-To: <20240306153703.499661d2@canb.auug.org.au>

On Tue, 5 Mar 2024 at 20:37, Stephen Rothwell <sfr@canb.auug.org.au> wrote:
>
> +static struct page *dump_page_copy(struct page *src, struct page *dst)
> +{
> +        return NULL;
> +}

No, it needs to be "return src;" not NULL.

That

  #define dump_page_copy(src, dst) ((dst), (src))

was supposed to be a "use 'dst', return 'src'" macro, and is correct
as that. The problem - as you noticed - is that it causes that "left
side of comma expression has no effect" warning.

(Technically it *does* have an effect - exactly the "argument is used"
one - but the compiler warning does make sense).

Actually, the simplest thing to do is probably just

  #define dump_page_free(x) ((void)(x))
  #define dump_page_copy(src, dst) (src)

where the "use" of the 'dump_page' argument is that dump_page_free()
void cast, and dump_page_copy() simply doesn't need to use it at all.

Christian?

            Linus