linux-kernel - Re: mmotm 2009-04-24-18-14 uploaded

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Message-ID: <57773.1240857915@turing-police.cc.vt.edu>
Date:	Mon, 27 Apr 2009 14:45:15 -0400
From:	Valdis.Kletnieks@...edu
To:	Nick Piggin <npiggin@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, mm-commits@...r.kernel.org
Subject: Re: mmotm 2009-04-24-18-14 uploaded - NVidia indigestion

On Sat, 25 Apr 2009 07:24:34 +0200, Nick Piggin said:

> >             #include <linux/version.h>
> >             #include <linux/utsname.h>
> >             int main() {
> >               if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,0)) {
> >                 return 0;
> >               } else {
> >                 return 1;
> >               }

> Yeah BUILD_BUG_ON does work like that. I can't for the life of me
> understand why it triggered though. Is anything in your external
> code calling kmalloc_slab directly?

See above - I didn't think LINUX_VERSION_CODE cared about kmalloc, but
I could be wrong on this. ;)

> What's the preprocessor output look like?

The problem appears to be that if you only include version.h and utsname.h,
and *don't* actually reference kmalloc_slab, it doesn't know that the size_t
is actually a constant, so it whinges.  Most modules end up doing at least
one kmalloc(), so kmalloc() and kmalloc_slab() get inlined, the constant 'size'
gets propogated, and life is good.  If you *don't* call kmalloc(), bad things
happen. ;)

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