Re: [PATCH V2] include: linux: Regularise the use of FIELD_SIZEOF macro

From: Andrew Morton <akpm@linux-foundation.org>
To: Andreas Dilger <adilger@dilger.ca>
Cc: Shyam Saini <shyam.saini@amarulasolutions.com>,
	kernel-hardening@lists.openwall.com,
	linux-kernel@vger.kernel.org, keescook@chromium.org,
	linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
	intel-gvt-dev@lists.freedesktop.org,
	intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
	netdev@vger.kernel.org, linux-ext4 <linux-ext4@vger.kernel.org>,
	devel@lists.orangefs.org, linux-mm@kvack.org,
	linux-sctp@vger.kernel.org, bpf@vger.kernel.org,
	kvm@vger.kernel.org, mayhs11saini@gmail.com,
	Alexey Dobriyan <adobriyan@gmail.com>
Subject: Re: [PATCH V2] include: linux: Regularise the use of FIELD_SIZEOF macro
Date: Tue, 11 Jun 2019 14:09:07 -0700	[thread overview]
Message-ID: <20190611140907.899bebb12a3d731da24a9ad1@linux-foundation.org> (raw)
In-Reply-To: <6DCAE4F8-3BEC-45F2-A733-F4D15850B7F3@dilger.ca>

On Tue, 11 Jun 2019 15:00:10 -0600 Andreas Dilger <adilger@dilger.ca> wrote:

> >> to FIELD_SIZEOF
> > 
> > As Alexey has pointed out, C structs and unions don't have fields -
> > they have members.  So this is an opportunity to switch everything to
> > a new member_sizeof().
> > 
> > What do people think of that and how does this impact the patch footprint?
> 
> I did a check, and FIELD_SIZEOF() is used about 350x, while sizeof_field()
> is about 30x, and SIZEOF_FIELD() is only about 5x.

Erk.  Sorry, I should have grepped.

> That said, I'm much more in favour of "sizeof_field()" or "sizeof_member()"
> than FIELD_SIZEOF().  Not only does that better match "offsetof()", with
> which it is closely related, but is also closer to the original "sizeof()".
> 
> Since this is a rather trivial change, it can be split into a number of
> patches to get approval/landing via subsystem maintainers, and there is no
> huge urgency to remove the original macros until the users are gone.  It
> would make sense to remove SIZEOF_FIELD() and sizeof_field() quickly so
> they don't gain more users, and the remaining FIELD_SIZEOF() users can be
> whittled away as the patches come through the maintainer trees.

In that case I'd say let's live with FIELD_SIZEOF() and remove
sizeof_field() and SIZEOF_FIELD().

I'm a bit surprised that the FIELD_SIZEOF() definition ends up in
stddef.h rather than in kernel.h where such things are normally
defined.  Why is that?