From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr1-f70.google.com (mail-wr1-f70.google.com [209.85.221.70]) by kanga.kvack.org (Postfix) with ESMTP id 9634D8E0002 for ; Wed, 16 Jan 2019 08:47:22 -0500 (EST) Received: by mail-wr1-f70.google.com with SMTP id 51so2931190wrb.15 for ; Wed, 16 Jan 2019 05:47:22 -0800 (PST) Received: from pegase1.c-s.fr (pegase1.c-s.fr. [93.17.236.30]) by mx.google.com with ESMTPS id q8si59679033wrf.2.2019.01.16.05.47.20 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 16 Jan 2019 05:47:21 -0800 (PST) Subject: Re: [PATCH V2] mm: Introduce GFP_PGTABLE References: <1547619692-7946-1-git-send-email-anshuman.khandual@arm.com> <20190116065703.GE24149@dhcp22.suse.cz> <20190116123018.GF6310@bombadil.infradead.org> <07d6a264-dccd-78ab-e8a9-2410bbef7b97@arm.com> <20190116131827.GH6310@bombadil.infradead.org> From: Christophe Leroy Message-ID: <521d8511-4c87-49c6-de03-67a71d5bacca@c-s.fr> Date: Wed, 16 Jan 2019 14:47:16 +0100 MIME-Version: 1.0 In-Reply-To: <20190116131827.GH6310@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox , Anshuman Khandual Cc: mark.rutland@arm.com, linux-sh@vger.kernel.org, peterz@infradead.org, catalin.marinas@arm.com, dave.hansen@linux.intel.com, will.deacon@arm.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux@armlinux.org.uk, mingo@redhat.com, vbabka@suse.cz, rientjes@google.com, palmer@sifive.com, greentime@andestech.com, marc.zyngier@arm.com, rppt@linux.vnet.ibm.com, shakeelb@google.com, kirill@shutemov.name, tglx@linutronix.de, Michal Hocko , linux-arm-kernel@lists.infradead.org, ard.biesheuvel@linaro.org, robin.murphy@arm.com, steve.capper@arm.com, christoffer.dall@arm.com, james.morse@arm.com, aneesh.kumar@linux.ibm.com, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org Le 16/01/2019 à 14:18, Matthew Wilcox a écrit : > On Wed, Jan 16, 2019 at 06:42:22PM +0530, Anshuman Khandual wrote: >> On 01/16/2019 06:00 PM, Matthew Wilcox wrote: >>> On Wed, Jan 16, 2019 at 07:57:03AM +0100, Michal Hocko wrote: >>>> On Wed 16-01-19 11:51:32, Anshuman Khandual wrote: >>>>> All architectures have been defining their own PGALLOC_GFP as (GFP_KERNEL | >>>>> __GFP_ZERO) and using it for allocating page table pages. This causes some >>>>> code duplication which can be easily avoided. GFP_KERNEL allocated and >>>>> cleared out pages (__GFP_ZERO) are required for page tables on any given >>>>> architecture. This creates a new generic GFP flag flag which can be used >>>>> for any page table page allocation. Does not cause any functional change. >>>>> >>>>> GFP_PGTABLE is being added into include/asm-generic/pgtable.h which is the >>>>> generic page tabe header just to prevent it's potential misuse as a general >>>>> allocation flag if included in include/linux/gfp.h. >>>> >>>> I haven't reviewed the patch yet but I am wondering whether this is >>>> really worth it without going all the way down to unify the common code >>>> and remove much more code duplication. Or is this not possible for some >>>> reason? >>> >>> Exactly what I suggested doing in response to v1. >>> >>> Also, the approach taken here is crazy. x86 has a feature that no other >>> architecture has bothered to implement yet -- accounting page tables >>> to the process. Yet instead of spreading that goodness to all other >>> architectures, Anshuman has gone to more effort to avoid doing that. >> >> The basic objective for this patch is to create a common minimum allocation >> flag that can be used by architectures but that still allows archs to add >> on additional constraints if they see fit. This patch does not intend to >> change functionality for any arch. > > I disagree with your objective. Making more code common is a great idea, > but this patch is too unambitious. We should be heading towards one or > two page table allocation functions instead of having every architecture do > its own thing. > > So start there. Move the x86 function into common code and convert one > other architecture to use it too. Are we talking about pte_alloc_one_kernel() and pte_alloc_one() ? I'm not sure x86 function is the best common one, as it seems to allocate a multiple of PAGE_SIZE only. Some arches like powerpc use pagetables which are smaller than a page, for instance powerpc 8xx uses 4k pagetables even with 16k pages, which means a single page can be used by 4 pagetables. Therefore, I would suggest to start with powerpc functions. Christophe