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[217.140.101.70]) by mx.google.com with ESMTP id s58si1579742edb.133.2019.03.01.05.39.36 for ; Fri, 01 Mar 2019 05:39:36 -0800 (PST) Received-SPF: pass (google.com: domain of steven.price@arm.com designates 217.140.101.70 as permitted sender) client-ip=217.140.101.70; Authentication-Results: mx.google.com; spf=pass (google.com: domain of steven.price@arm.com designates 217.140.101.70 as permitted sender) smtp.mailfrom=steven.price@arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 6111CA78; Fri, 1 Mar 2019 05:39:35 -0800 (PST) Received: from [10.1.196.69] (e112269-lin.cambridge.arm.com [10.1.196.69]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 19B2E3F5C1; Fri, 1 Mar 2019 05:39:31 -0800 (PST) Subject: Re: [PATCH v2 03/13] mm: Add generic p?d_large() macros To: "Kirill A. Shutemov" , Mike Rapoport Cc: Mark Rutland , x86@kernel.org, Arnd Bergmann , Ard Biesheuvel , Peter Zijlstra , Catalin Marinas , Dave Hansen , Will Deacon , 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 , linux-arm-kernel@lists.infradead.org, "Liang, Kan" References: <20190221113502.54153-1-steven.price@arm.com> <20190221113502.54153-4-steven.price@arm.com> <20190221142812.oa53lfnnfmsuh6ys@kshutemo-mobl1> <20190221145706.zqwfdoyiirn3lc7y@kshutemo-mobl1> <20190221210618.voyfs5cnafpvgedh@kshutemo-mobl1> <20190301115300.GE5156@rapoport-lnx> <20190301123031.rw3dswcoaa2x7haq@kshutemo-mobl1> From: Steven Price Message-ID: Date: Fri, 1 Mar 2019 13:39:30 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.0 MIME-Version: 1.0 In-Reply-To: <20190301123031.rw3dswcoaa2x7haq@kshutemo-mobl1> 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 01/03/2019 12:30, Kirill A. Shutemov wrote: > On Fri, Mar 01, 2019 at 01:53:01PM +0200, Mike Rapoport wrote: >> Him Kirill, >> >> On Fri, Feb 22, 2019 at 12:06:18AM +0300, Kirill A. Shutemov wrote: >>> On Thu, Feb 21, 2019 at 05:16:46PM +0000, Steven Price wrote: >>>>>> Note that in terms of the new page walking code, these new defines are >>>>>> only used when walking a page table without a VMA (which isn't currently >>>>>> done), so architectures which don't use p?d_large currently will work >>>>>> fine with the generic versions. They only need to provide meaningful >>>>>> definitions when switching to use the walk-without-a-VMA functionality. >>>>> >>>>> How other architectures would know that they need to provide the helpers >>>>> to get walk-without-a-VMA functionality? This looks very fragile to me. >>>> >>>> Yes, you've got a good point there. This would apply to the p?d_large >>>> macros as well - any arch which (inadvertently) uses the generic version >>>> is likely to be fragile/broken. >>>> >>>> I think probably the best option here is to scrap the generic versions >>>> altogether and simply introduce a ARCH_HAS_PXD_LARGE config option which >>>> would enable the new functionality to those arches that opt-in. Do you >>>> think this would be less fragile? >>> >>> These helpers are useful beyond pagewalker. >>> >>> Can we actually do some grinding and make *all* archs to provide correct >>> helpers? Yes, it's tedious, but not that bad. >> >> Many architectures simply cannot support non-leaf entries at the higher >> levels. I think letting the use a generic helper actually does make sense. > > I disagree. > > It's makes sense if the level doesn't exists on the arch. This is what patch 24 [1] of the series does - if the level doesn't exist then appropriate stubs are provided. > But if the level exists, it will be less frugile to ask the arch to > provide the helper. Even if it is dummy always-false. The problem (as I see it), is we need a reliable set of p?d_large() implementations to be able to walk arbitrary page tables. Either the entire functionality of walking page tables without a VMA has to be an opt-in per architecture, or we need to mandate that every architecture provide these implementations. I could provide an asm-generic header to provide a complete set of dummy implementations for architectures that don't support large pages at all, but that seems a bit overkill when most architectures only need to define 2 or 3 implementations (the rest being provided by the folded-levels automatically). Thanks, Steve [1] https://lore.kernel.org/lkml/20190227170608.27963-25-steven.price@arm.com/