Date: Fri, 25 Aug 2017 08:45:40 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Russell King - ARM Linux <linux@armlinux.org.uk>
Cc: LKML <linux-kernel@vger.kernel.org>
Subject: Re: arch/arm/kernel/setup.c fails to compile for NOMMU
Message-ID: <20170825064540.GB25498@dhcp22.suse.cz>
References: <20170818112402.GB18499@dhcp22.suse.cz>
 <20170824161741.GX20805@n2100.armlinux.org.uk>
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On Thu 24-08-17 17:17:41, Russell King - ARM Linux wrote:
> On Fri, Aug 18, 2017 at 01:24:02PM +0200, Michal Hocko wrote:
> > Hi Russel,
> > I have a battery of configs for compile testing and for some time I've
> > been seeing the following compilation error with nommu config (attached)
> > 
> > arch/arm/kernel/setup.c: In function 'reserve_crashkernel':
> > arch/arm/kernel/setup.c:1005:25: error: 'SECTION_SIZE' undeclared (first
> > use in this function)
> >              crash_size, SECTION_SIZE);
> > 
> > I didn't get to look what is going on here, maybe my config is just too
> > artificial but the primary reason is that SECTION_SIZE is not defined in
> > pgtable-nommu.h. To be honest I am not familiar with nommu very much and
> > it smells like the whole reserve_crashkernel doesn't really make any
> > sense on those configs. Could you have a look what is the best fix
> > please?
> 
> Hi,
> 
> I suspect that mach-netx has never been tested in nommu configurations
> (ditto for many of the older platforms, which pre-date merging nommu
> support.)
> 
> Maybe the best solution is to make these old platforms depend on MMU.
> 
> However, I'm wondering whether kexec makes sense for !MMU - that's
> probably something that hasn't been tested and doesn't actually work.
> So maybe another approach would be to make kexec depend on MMU for
> ARM - but I'm afraid I don't really know.

Yeah, I've disabled KEXEC in my testing config. All I do care about is
to test nommu specific code paths in MM code.

> I only have very limited nommu experience.

me too

So what would you say about the following?
---
>From 2707f3bf00181bbc9dcf6a1f287eb7369141e955 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Fri, 25 Aug 2017 08:40:09 +0200
Subject: [PATCH] arm: make kexec depend on MMU

arm nommu config with KEXEC enabled doesn't compile
arch/arm/kernel/setup.c: In function 'reserve_crashkernel':
arch/arm/kernel/setup.c:1005:25: error: 'SECTION_SIZE' undeclared (first
use in this function)
             crash_size, SECTION_SIZE);

since 61603016e212 ("ARM: kexec: fix crashkernel= handling") which is
over one year without anybody noticing. I have only noticed beause of
my testing nommu config which somehow gained CONFIG_KEXEC without
an intention. This suggests that nobody is actually using KEXEC
on nommu ARM configs. It is even a question whether kexec works with
nommu.

Make KEXEC depend on MMU to make this clear. If somebody wants to enable
there will be probably more things to take care.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 arch/arm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 3f4aa9179337..c8603195d7fc 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -2003,6 +2003,7 @@ config KEXEC
 	bool "Kexec system call (EXPERIMENTAL)"
 	depends on (!SMP || PM_SLEEP_SMP)
 	depends on !CPU_V7M
+	depends on MMU
 	select KEXEC_CORE
 	help
 	  kexec is a system call that implements the ability to shutdown your
-- 
2.13.2

-- 
Michal Hocko
SUSE Labs