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From:   Shyam Saini <mayhs11saini@gmail.com>
Date:   Wed, 12 Jun 2019 02:57:58 +0530
Message-ID: <CAOfkYf5_HTN1HO0gQY9iGchK5Anf6oVx7knzMhL1hWpv4gV20Q@mail.gmail.com>
Subject: Re: [PATCH V2] include: linux: Regularise the use of FIELD_SIZEOF macro
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Andreas Dilger <adilger@dilger.ca>,
        Shyam Saini <shyam.saini@amarulasolutions.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Kees Cook <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 <dri-devel@lists.freedesktop.org>,
        Network Development <netdev@vger.kernel.org>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        devel@lists.orangefs.org, linux-mm <linux-mm@kvack.org>,
        linux-sctp@vger.kernel.org, bpf <bpf@vger.kernel.org>,
        kvm@vger.kernel.org, Alexey Dobriyan <adobriyan@gmail.com>
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Hi Andrew,

>
> 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?

Thanks for pointing out this, I was not aware if this is a convention.
Anyway, I'll keep FIELD_SIZEOF definition in kernel.h in next version.