linux-kernel - Re: [PATCH 00/17] Add memberof(), split some headers, and slightly simplify code

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Date:   Fri, 19 Nov 2021 16:06:27 +0100
From:   "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ajit Khaparde <ajit.khaparde@...adcom.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Borislav Petkov <bp@...e.de>,
        Corey Minyard <cminyard@...sta.com>, Chris Mason <clm@...com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        David Sterba <dsterba@...e.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Jason Wang <jasowang@...hat.com>,
        Jitendra Bhivare <jitendra.bhivare@...adcom.com>,
        John Hubbard <jhubbard@...dia.com>,
        "John S . Gruber" <JohnSGruber@...il.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Josef Bacik <josef@...icpanda.com>,
        Kees Cook <keescook@...omium.org>,
        Ketan Mukadam <ketan.mukadam@...adcom.com>,
        Len Brown <lenb@...nel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Russell King <linux@...linux.org.uk>,
        Somnath Kotur <somnath.kotur@...adcom.com>,
        Sriharsha Basavapatna <sriharsha.basavapatna@...adcom.com>,
        Subbu Seetharaman <subbu.seetharaman@...adcom.com>,
        intel-gfx@...ts.freedesktop.org, linux-acpi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-btrfs@...r.kernel.org,
        linux-scsi@...r.kernel.org, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 00/17] Add memberof(), split some headers, and slightly
 simplify code

Hi Arnd,

On 11/19/21 15:47, Arnd Bergmann wrote:
> On Fri, Nov 19, 2021 at 12:36 PM Alejandro Colomar
> <alx.manpages@...il.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.

Yes, I would like to untangle the dependencies.

The main reason I started doing this splitting
is because I wouldn't be able to include
<linux/stddef.h> in some headers,
because it pulled too much stuff that broke unrelated things.

So that's why I started from there.

I for example would like to get NULL in memberof()
without puling anything else,
so <linux/NULL.h> makes sense for that.

It's clear that every .c wants NULL,
but it's not so clear that every .c wants
everything that <linux/stddef.h> pulls indirectly.

But I'll note that linux/fs.h, linux/sched.h, linux/mm.h are
interesting headers for further splitting.


BTW, I also have a longstanding doubt about
how header files are organized in the kernel,
and which headers can and cannot be included
from which other files.

For example I see that files in samples or scripts or tools,
that redefine many things such as offsetof() or ARRAY_SIZE(),
and I don't know if there's a good reason for that,
or if I should simply remove all that stuff and
include <linux/offsetof.h> everywhere I see offsetof() being used.



Thanks,
Alex

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/