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Subject: Re: [PATCH V2] mm/ioremap: Probe platform for p4d huge map support
To: Andrew Morton <akpm@linux-foundation.org>
References: <1561699231-20991-1-git-send-email-anshuman.khandual@arm.com>
 <20190702160630.25de5558e9fe2d7d845f3472@linux-foundation.org>
From: Anshuman Khandual <anshuman.khandual@arm.com>
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Date: Thu, 4 Jul 2019 20:56:40 +0530
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Cc: Stephen Rothwell <sfr@canb.auug.org.au>, x86@kernel.org,
 Peter Zijlstra <peterz@infradead.org>,
 Catalin Marinas <catalin.marinas@arm.com>,
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 Andy Lutomirski <luto@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
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On 07/03/2019 04:36 AM, Andrew Morton wrote:
> On Fri, 28 Jun 2019 10:50:31 +0530 Anshuman Khandual <anshuman.khandual@arm.com> wrote:
> 
>> Finishing up what the commit c2febafc67734a ("mm: convert generic code to
>> 5-level paging") started out while levelling up P4D huge mapping support
>> at par with PUD and PMD. A new arch call back arch_ioremap_p4d_supported()
>> is being added which just maintains status quo (P4D huge map not supported)
>> on x86, arm64 and powerpc.
> 
> Does this have any runtime effects?  If so, what are they and why?  If
> not, what's the actual point?

It just finishes up what the previous commit c2febafc67734a ("mm: convert
generic code to 5-level paging") left off with respect p4d based huge page
enablement for ioremap. When HAVE_ARCH_HUGE_VMAP is enabled its just a simple
check from the arch about the support, hence runtime effects are minimal.