linux-kernel - Re: [PATCH v3] arm64: add ARM64-specific support for flatmem

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Date:   Tue, 10 Jul 2018 15:16:37 +0100
From:   Will Deacon <will.deacon@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nikunj Kela <nkela@...co.com>,
        Catalin Marinas <catalin.marinas@....com>,
        xe-kernel@...ernal.cisco.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3] arm64: add ARM64-specific support for flatmem

On Tue, Jul 10, 2018 at 03:25:14PM +0200, Arnd Bergmann wrote:
> On Tue, Jul 10, 2018 at 3:06 PM, Will Deacon <will.deacon@....com> wrote:
> > On Tue, Jul 10, 2018 at 02:55:41PM +0200, Arnd Bergmann wrote:
> >> On Fri, Jul 6, 2018 at 7:47 PM, Nikunj Kela <nkela@...co.com> wrote:
> >> > Flatmem is useful in reducing kernel memory usage.
> >> > One usecase is in kdump kernel. We are able to save
> >> > ~14M by moving to flatmem scheme.
> >> >
> >> > Cc: xe-kernel@...ernal.cisco.com
> >> > Cc: Nikunj Kela <nkela@...co.com>
> >> > Signed-off-by: Nikunj Kela <nkela@...co.com>
> >> > ---
> >> >  arch/arm64/Kconfig | 3 +++
> >> >  1 file changed, 3 insertions(+)
> >> >
> >> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> >> > index 42c090c..f5b4c49 100644
> >> > --- a/arch/arm64/Kconfig
> >> > +++ b/arch/arm64/Kconfig
> >> > @@ -775,6 +775,9 @@ config ARCH_SPARSEMEM_DEFAULT
> >> >  config ARCH_SELECT_MEMORY_MODEL
> >> >         def_bool ARCH_SPARSEMEM_ENABLE
> >> >
> >> > +config ARCH_FLATMEM_ENABLE
> >> > +       def_bool y
> >> > +
> >> >  config HAVE_ARCH_PFN_VALID
> >> >         def_bool ARCH_HAS_HOLES_MEMORYMODEL || !SPARSEMEM
> >> >
> >>
> >> I'm getting hundreds of errors in randconfig builds with this:
> >
> > Damn, I was worried something like this might happen. Sorry!
> >
> > Does randconfig also fuzz CONFIG_EXPERT? We only added ARCH_FLATMEM_ENABLE
> > so that people can hand-configure crashkernels to make them small; there will
> > be plenty of situations where it's just going to cause more problems than it
> > solves, so I'd be happy hiding it behind EXPERT if it helps.
> 
> Yes, randconfig also tests EXPERT and COMPILE_TEST configurations, though I
> don't test the !COMPILE_TEST ones myself, so you can also hide options from
> me by making them 'depends on !COMPILE_TEST'.
> 
> > Otherwise we can probably through a 'def_bool !NUMA' at it.
> 
> Yes, I was going to send a patch with that next after testing it some more.
> So far it's holding up. This is also what some other architectures have, so
> it's probably safe.

Ok, let's go with that then. If I don't see a patch from you in a day or
two, I'll write one myself :)

Will