[PATCH] Extract initrd free logic from arch-specific code.

[linux@armlinux.org.uk (Russell King - ARM Linux)]

On Thu, Mar 29, 2018 at 09:37:52AM +1100, Oliver wrote:
> On Thu, Mar 29, 2018 at 9:14 AM, Russell King - ARM Linux
> <linux@armlinux.org.uk> wrote:
> > On Wed, Mar 28, 2018 at 02:04:22PM -0500, Rob Landley wrote:
> >>
> >>
> >> On 03/28/2018 11:48 AM, Russell King - ARM Linux wrote:
> >> > On Wed, Mar 28, 2018 at 10:58:51AM -0500, Rob Landley wrote:
> >> >> On 03/28/2018 10:26 AM, Shea Levy wrote:
> >> >>> Now only those architectures that have custom initrd free requirements
> >> >>> need to define free_initrd_mem.
> >> >> ...
> >> >>> --- a/arch/arc/mm/init.c
> >> >>> +++ b/arch/arc/mm/init.c
> >> >>> @@ -229,10 +229,3 @@ void __ref free_initmem(void)
> >> >>>  {
> >> >>>   free_initmem_default(-1);
> >> >>>  }
> >> >>> -
> >> >>> -#ifdef CONFIG_BLK_DEV_INITRD
> >> >>> -void __init free_initrd_mem(unsigned long start, unsigned long end)
> >> >>> -{
> >> >>> - free_reserved_area((void *)start, (void *)end, -1, "initrd");
> >> >>> -}
> >> >>> -#endif
> >> >>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> >> >>> index 3f972e83909b..19d1c5594e2d 100644
> >> >>> --- a/arch/arm/Kconfig
> >> >>> +++ b/arch/arm/Kconfig
> >> >>> @@ -47,6 +47,7 @@ config ARM
> >> >>>   select HARDIRQS_SW_RESEND
> >> >>>   select HAVE_ARCH_AUDITSYSCALL if (AEABI && !OABI_COMPAT)
> >> >>>   select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
> >> >>> + select HAVE_ARCH_FREE_INITRD_MEM
> >> >>>   select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU
> >> >>>   select HAVE_ARCH_KGDB if !CPU_ENDIAN_BE32 && MMU
> >> >>>   select HAVE_ARCH_MMAP_RND_BITS if MMU
> >> >>
> >> >> Isn't this why weak symbols were invented?
> >> >
> >> > Weak symbols means that we end up with both the weakly-referenced code
> >> > and the arch code in the kernel image.  That's fine if the weak code
> >> > is small.
> >>
> >> The kernel's been able to build with link time garbage collection since 2016:
> >>
> >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b67067f1176d
> >>
> >> Wouldn't that remove the unused one?
> >
> > Probably, if anyone bothered to use that, which they don't.
> >
> > LD_DEAD_CODE_DATA_ELIMINATION is a symbol without a prompt, and from
> > what I can see, nothing selects it.  Therefore, the symbol is always
> > disabled, and so the feature never gets used in mainline kernels.
> >
> > Brings up the obvious question - why is it there if it's completely
> > unused?  (Maybe to cause confusion, and allowing a justification
> > for __weak ?)
> 
> IIRC Nick had some patches to do the arch enablement for powerpc, but
> I'm not sure what happened to them though. I suspect it just fell down
> Nick's ever growing TODO list.

I've given it a go on ARM, marking every linker-built table with KEEP()
and comparing the System.map files.  The resulting kernel is around
150k smaller, which seems good.

However, it doesn't boot - and I don't know why.  Booting the kernel
under kvmtool in a VM using virtio-console, I can find no way to get
any kernel messages out of it.  Using lkvm debug, I can see that the
PC is stuck inside die(), and that's the only information I have.
It dies before bringing up the other CPUs, so it's a very early death.

I don't think other console types are available under ARM64.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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