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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 19 Nov 2021 15:47:35 +0100
Message-ID: <CAK8P3a0qT9tAxFkLN_vJYRcocDW2TcBq79WcYKZFyAG0udZx5Q@mail.gmail.com>
Subject: Re: [PATCH 00/17] Add memberof(), split some headers, and slightly
 simplify code
To:     Alejandro Colomar <alx.manpages@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Ajit Khaparde <ajit.khaparde@broadcom.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Borislav Petkov <bp@suse.de>,
        Corey Minyard <cminyard@mvista.com>, Chris Mason <clm@fb.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        David Sterba <dsterba@suse.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Jason Wang <jasowang@redhat.com>,
        Jitendra Bhivare <jitendra.bhivare@broadcom.com>,
        John Hubbard <jhubbard@nvidia.com>,
        "John S . Gruber" <JohnSGruber@gmail.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
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        Kees Cook <keescook@chromium.org>,
        Ketan Mukadam <ketan.mukadam@broadcom.com>,
        Len Brown <lenb@kernel.org>,
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        Miguel Ojeda <ojeda@kernel.org>,
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On Fri, Nov 19, 2021 at 12:36 PM Alejandro Colomar
<alx.manpages@gmail.com> wrote:
>
> Alejandro Colomar (17):
>   linux/container_of.h: Add memberof(T, m)
>   Use memberof(T, m) instead of explicit NULL dereference
>   Replace some uses of memberof() by its wrappers
>   linux/memberof.h: Move memberof() to separate header
>   linux/typeof_member.h: Move typeof_member() to a separate header
>   Simplify sizeof(typeof_member()) to sizeof_field()
>   linux/NULL.h: Move NULL to a separate header
>   linux/offsetof.h: Move offsetof(T, m) to a separate header
>   linux/offsetof.h: Implement offsetof() in terms of memberof()
>   linux/container_of.h: Implement container_of_safe() in terms of
>     container_of()
>   linux/container_of.h: Cosmetic
>   linux/container_of.h: Remove unnecessary cast to (void *)

My feeling is that this takes the separation too far: by having this many header
files that end up being included from practically every single .c file
in the kernel,
I think you end up making compile speed worse overall.

If your goal is to avoid having to recompile as much of the kernel
after touching
a header, I think a better approach is to help untangle the dependencies, e.g.
by splitting out type definitions from headers with inline functions (most
indirect header dependencies are on type definitions) and by focusing on
linux/fs.h, linux/sched.h, linux/mm.h and how they interact with the rest of the
headers. At the moment, these are included in most .c files and they in turn
include a ton of other headers.

          Arnd