linux-kernel - Re: [PATCH v1 1/1] kernel.h: Drop inclusion in bitmap.h

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Date:   Sun, 28 Mar 2021 00:38:00 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yury Norov <yury.norov@...il.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH v1 1/1] kernel.h: Drop inclusion in bitmap.h

On Sun, Mar 28, 2021 at 12:23 AM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> On Fri, Mar 26, 2021 at 07:03:47PM +0200, Andy Shevchenko wrote:
> > The bitmap.h header is used in a lot of code around the kernel.
> > Besides that it includes kernel.h which sometimes makes a loop.
>
> How much of the kernel does *not* end up pulling kernel.h anyway,
> making all subsequent includes fast no-ops?
>
> Realistically, any C compiler is going to recognize the case when
> included file has contents of form
>
> #ifndef <pp-id>
> #define <pp-id> <possibly empty sequence of preprocessor tokens>
> <lines>
> #endif
>
> where <lines> contain no exposed #else, #elif or #endif and
> remember that subsequent includes of that file should be ignored
> unless <pp-id> becomes undefined.

The problem here is not in C compiler, but in many unneeded loops that
make header hell dependencies.
For example, how you may move bitmap_zalloc() from C-file to the
header? Currently it's impossible.
And bitmap.h here is only a tip of an iceberg.

kerne.h is a dump of everything that even has nothing in common at all.
We may still have it, but in my new code I prefer to include only the
headers that I want to use, without the bulk of unneeded kernel code.

> AFAICS, on e.g. allmodconfig amd64 build it's (at most - there
> might've been false positives) 131 files out of ~30000; on
> defconfig it's 55 out of ~2300.  How much does your patch
> change that?

I'm not sure I understand what you mean here.

-- 
With Best Regards,
Andy Shevchenko