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

Alejandro Colomar (man-pages)
2021-11-19T15:06:34Z

Hi Arnd,

On 11/19/21 15:47, Arnd Bergmann wrote:
> 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.

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/