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[217.140.110.172]) by mx.google.com with ESMTP id gq12si21873265ejb.170.2019.08.01.05.22.35 for ; Thu, 01 Aug 2019 05:22:35 -0700 (PDT) Received-SPF: pass (google.com: domain of steven.price@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 steven.price@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=steven.price@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 879161570; Thu, 1 Aug 2019 05:22:34 -0700 (PDT) Received: from [10.1.196.133] (e112269-lin.cambridge.arm.com [10.1.196.133]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EF0683F575; Thu, 1 Aug 2019 05:22:31 -0700 (PDT) Subject: Re: [PATCH v9 10/21] mm: Add generic p?d_leaf() macros To: Anshuman Khandual , Mark Rutland Cc: Dave Hansen , Arnd Bergmann , Ard Biesheuvel , Peter Zijlstra , Catalin Marinas , x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Ingo Molnar , Borislav Petkov , Andy Lutomirski , "H. Peter Anvin" , James Morse , Thomas Gleixner , Will Deacon , Andrew Morton , linux-arm-kernel@lists.infradead.org, "Liang, Kan" References: <20190722154210.42799-1-steven.price@arm.com> <20190722154210.42799-11-steven.price@arm.com> <20190723094113.GA8085@lakrids.cambridge.arm.com> <674bd809-f853-adb0-b1ab-aa4404093083@arm.com> <0979d4b4-7a97-2dc3-67cf-3aa6569bfdcd@arm.com> From: Steven Price Message-ID: Date: Thu, 1 Aug 2019 13:22:30 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <0979d4b4-7a97-2dc3-67cf-3aa6569bfdcd@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB 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 01/08/2019 07:09, Anshuman Khandual wrote: > > > On 07/29/2019 05:08 PM, Steven Price wrote: >> On 28/07/2019 12:44, Anshuman Khandual wrote: >>> >>> >>> On 07/23/2019 03:11 PM, Mark Rutland wrote: >>>> On Mon, Jul 22, 2019 at 04:41:59PM +0100, Steven Price wrote: >>>>> Exposing the pud/pgd levels of the page tables to walk_page_range() means >>>>> we may come across the exotic large mappings that come with large areas >>>>> of contiguous memory (such as the kernel's linear map). >>>>> >>>>> For architectures that don't provide all p?d_leaf() macros, provide >>>>> generic do nothing default that are suitable where there cannot be leaf >>>>> pages that that level. >>>>> >>>>> Signed-off-by: Steven Price >>>> >>>> Not a big deal, but it would probably make sense for this to be patch 1 >>>> in the series, given it defines the semantic of p?d_leaf(), and they're >>>> not used until we provide all the architectural implemetnations anyway. >>> >>> Agreed. >>> >>>> >>>> It might also be worth pointing out the reasons for this naming, e.g. >>>> p?d_large() aren't currently generic, and this name minimizes potential >>>> confusion between p?d_{large,huge}(). >>> >>> Agreed. But these fallback also need to first check non-availability of large >>> pages. I am not sure whether CONFIG_HUGETLB_PAGE config being clear indicates >>> that conclusively or not. Being a page table leaf entry has a broader meaning >>> than a large page but that is really not the case today. All leaf entries here >>> are large page entries from MMU perspective. This dependency can definitely be >>> removed when there are other types of leaf entries but for now IMHO it feels >>> bit problematic not to directly associate leaf entries with large pages in >>> config restriction while doing exactly the same. >> >> The intention here is that the page walkers are able to walk any type of >> page table entry which the kernel may use. CONFIG_HUGETLB_PAGE only >> controls whether "huge TLB pages" are used by user space processes. It's >> quite possible that option to not be selected but the linear mapping to >> have been mapped using "large pages" (i.e. leaf entries further up the >> tree than normal). > > I understand that kernel page table might use large pages where as user space > never enabled HugeTLB. The point to make here was CONFIG_HUGETLB approximately > indicates the presence of large pages though the absence of same does not > conclusively indicate that large pages are really absent on the MMU. Perhaps it > will requires something new like MMU_[LARGE|HUGE]_PAGES. CONFIG_HUGETLB doesn't necessarily mean leaf entries can appear anywhere other than PTE. Some architectures always have a full tree of page tables, but can program their TLBs with larger entries - I think all the architectures I've come across have software page table walking, but in theory the arm64 contiguous hint bit could be considered similar. Steve