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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 10 Jul 2018 15:25:14 +0200
Message-ID: <CAK8P3a2ss5tYve6xL7eE50ev3zVPOHzN5QtroWYmTih0F3sQYQ@mail.gmail.com>
Subject: Re: [PATCH v3] arm64: add ARM64-specific support for flatmem
To:     Will Deacon <will.deacon@arm.com>
Cc:     Nikunj Kela <nkela@cisco.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        xe-kernel@external.cisco.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
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On Tue, Jul 10, 2018 at 3:06 PM, Will Deacon <will.deacon@arm.com> wrote:
> Hi Arnd,
>
> 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@cisco.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@external.cisco.com
>> > Cc: Nikunj Kela <nkela@cisco.com>
>> > Signed-off-by: Nikunj Kela <nkela@cisco.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.

     Arnd