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[217.140.110.172]) by mx.google.com with ESMTP id b2si2742624ejq.303.2019.07.12.02.57.55 for ; Fri, 12 Jul 2019 02:57:55 -0700 (PDT) Received-SPF: pass (google.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) client-ip=217.140.110.172; Authentication-Results: mx.google.com; spf=pass (google.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 054A42B; Fri, 12 Jul 2019 02:57:55 -0700 (PDT) Received: from [10.162.41.115] (p8cg001049571a15.blr.arm.com [10.162.41.115]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C4A683F71F; Fri, 12 Jul 2019 02:57:50 -0700 (PDT) Subject: Re: [PATCH V2] mm/ioremap: Probe platform for p4d huge map support To: Michael Ellerman , Andrew Morton Cc: linux-mm@kvack.org, Catalin Marinas , Will Deacon , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , "Kirill A . Shutemov" , Michal Hocko , Stephen Rothwell , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, x86@kernel.org References: <1561699231-20991-1-git-send-email-anshuman.khandual@arm.com> <20190702160630.25de5558e9fe2d7d845f3472@linux-foundation.org> <87tvbrennf.fsf@concordia.ellerman.id.au> From: Anshuman Khandual Message-ID: Date: Fri, 12 Jul 2019 15:28:22 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <87tvbrennf.fsf@concordia.ellerman.id.au> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 07/12/2019 12:37 PM, Michael Ellerman wrote: > Anshuman Khandual writes: >> On 07/03/2019 04:36 AM, Andrew Morton wrote: >>> On Fri, 28 Jun 2019 10:50:31 +0530 Anshuman Khandual 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. > > The return value of arch_ioremap_p4d_supported() is stored in the > variable ioremap_p4d_capable which is then returned by > ioremap_p4d_enabled(). > > That is used by ioremap_try_huge_p4d() called from ioremap_p4d_range() > from ioremap_page_range(). That is right. > > The runtime effect is that it prevents ioremap_page_range() from trying > to create huge mappings at the p4d level on arches that don't support > it. But now after first checking with an arch callback. Previously p4d huge mappings were disabled on all platforms as ioremap_p4d_capable remained clear through out being a static.