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

From: Andreas Dilger <adilger@dilger.ca>
To: Andrew Morton <akpm@linux-foundation.org>
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 15:00:10 -0600	[thread overview]
Message-ID: <6DCAE4F8-3BEC-45F2-A733-F4D15850B7F3@dilger.ca> (raw)
In-Reply-To: <20190611134831.a60c11f4b691d14d04a87e29@linux-foundation.org>

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On Jun 11, 2019, at 2:48 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> On Wed, 12 Jun 2019 01:08:36 +0530 Shyam Saini <shyam.saini@amarulasolutions.com> wrote:
> 
>> Currently, there are 3 different macros, namely sizeof_field, SIZEOF_FIELD
>> and FIELD_SIZEOF which are used to calculate the size of a member of
>> structure, so to bring uniformity in entire kernel source tree lets use
>> FIELD_SIZEOF and replace all occurrences of other two macros with this.
>> 
>> For this purpose, redefine FIELD_SIZEOF in include/linux/stddef.h and
>> tools/testing/selftests/bpf/bpf_util.h and remove its defination from
>> include/linux/kernel.h
>> 
>> In favour of FIELD_SIZEOF, this patch also deprecates other two similar
>> macros sizeof_field and SIZEOF_FIELD.
>> 
>> For code compatibility reason, retain sizeof_field macro as a wrapper macro
>> 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.

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.

Cheers, Andreas

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