From mboxrd@z Thu Jan 1 00:00:00 1970 From: joel@joelfernandes.org (Joel Fernandes) Date: Sat, 27 Oct 2018 12:39:17 -0700 Subject: [PATCH 2/4] mm: speed up mremap by 500x on large regions (v2) In-Reply-To: <20181027102102.GO8537@350D> References: <20181013013200.206928-1-joel@joelfernandes.org> <20181013013200.206928-3-joel@joelfernandes.org> <20181024101255.it4lptrjogalxbey@kshutemo-mobl1> <20181024115733.GN8537@350D> <20181025021350.GB13560@joelaf.mtv.corp.google.com> <20181027102102.GO8537@350D> Message-ID: <20181027193917.GA51131@joelaf.mtv.corp.google.com> To: linux-riscv@lists.infradead.org List-Id: linux-riscv.lists.infradead.org Hi Balbir, On Sat, Oct 27, 2018 at 09:21:02PM +1100, Balbir Singh wrote: > On Wed, Oct 24, 2018 at 07:13:50PM -0700, Joel Fernandes wrote: > > On Wed, Oct 24, 2018 at 10:57:33PM +1100, Balbir Singh wrote: > > [...] > > > > > + pmd_t pmd; > > > > > + > > > > > + new_ptl = pmd_lockptr(mm, new_pmd); > > > > > > > > > Looks like this is largely inspired by move_huge_pmd(), I guess a lot of > > > the code applies, why not just reuse as much as possible? The same comments > > > w.r.t mmap_sem helping protect against lock order issues applies as well. > > > > I thought about this and when I looked into it, it seemed there are subtle > > differences that make such sharing not worth it (or not possible). > > > > Could you elaborate on them? The move_huge_page function is defined only for CONFIG_TRANSPARENT_HUGEPAGE so we cannot reuse it to begin with, since we have it disabled on our systems. I am not sure if it is a good idea to split that out and refactor it for reuse especially since our case is quite simple compared to huge pages. There are also a couple of subtle differences between the move_normal_pmd and the move_huge_pmd. Atleast 2 of them are: 1. We don't concern ourself with the PMD dirty bit, since the pages being moved are normal pages and at the soft-dirty bit accounting is at the PTE level, since we are not moving PTEs, we don't need to do that. 2. The locking is simpler as Kirill pointed, pmd_lock cannot fail however __pmd_trans_huge_lock can. I feel it is not super useful to refactor move_huge_pmd to support our case especially since move_normal_pmd is quite small, so IMHO the benefit of code reuse isn't there very much. Do let me know your thoughts and thanks for your interest in this. thanks, - Joel From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 10870C46475 for ; Sat, 27 Oct 2018 19:39:40 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id BE4E92082C for ; Sat, 27 Oct 2018 19:39:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="QYUYTxPb"; 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Sat, 27 Oct 2018 12:39:18 -0700 (PDT) Date: Sat, 27 Oct 2018 12:39:17 -0700 From: Joel Fernandes To: Balbir Singh Subject: Re: [PATCH 2/4] mm: speed up mremap by 500x on large regions (v2) Message-ID: <20181027193917.GA51131@joelaf.mtv.corp.google.com> References: <20181013013200.206928-1-joel@joelfernandes.org> <20181013013200.206928-3-joel@joelfernandes.org> <20181024101255.it4lptrjogalxbey@kshutemo-mobl1> <20181024115733.GN8537@350D> <20181025021350.GB13560@joelaf.mtv.corp.google.com> <20181027102102.GO8537@350D> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20181027102102.GO8537@350D> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20181027_123931_518438_8EAA1CBC X-CRM114-Status: GOOD ( 17.18 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-mips@linux-mips.org, Rich Felker , linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, Peter Zijlstra , Catalin Marinas , Dave Hansen , Will Deacon , mhocko@kernel.org, linux-mm@kvack.org, lokeshgidra@google.com, sparclinux@vger.kernel.org, linux-riscv@lists.infradead.org, elfring@users.sourceforge.net, Jonas Bonn , kvmarm@lists.cs.columbia.edu, dancol@google.com, Yoshinori Sato , linux-xtensa@linux-xtensa.org, linux-hexagon@vger.kernel.org, Helge Deller , "maintainer:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" , hughd@google.com, "James E.J. Bottomley" , kasan-dev@googlegroups.com, anton.ivanov@kot-begemot.co.uk, Ingo Molnar , Geert Uytterhoeven , Andrey Ryabinin , linux-snps-arc@lists.infradead.org, kernel-team@android.com, Sam Creasey , Fenghua Yu , linux-s390@vger.kernel.org, Jeff Dike , linux-um@lists.infradead.org, Stefan Kristiansson , Julia Lawall , linux-m68k@lists.linux-m68k.org, Borislav Petkov , Andy Lutomirski , nios2-dev@lists.rocketboards.org, "Kirill A. Shutemov" , Stafford Horne , Guan Xuetao , Chris Zankel , Tony Luck , Richard Weinberger , linux-parisc@vger.kernel.org, pantin@google.com, Max Filippov , linux-kernel@vger.kernel.org, minchan@kernel.org, Thomas Gleixner , linux-alpha@vger.kernel.org, Ley Foon Tan , akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org, "David S. Miller" Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+infradead-linux-riscv=archiver.kernel.org@lists.infradead.org Message-ID: <20181027193917.4bi4eH0QalqrYLVQkfQF3Ssxcg4If9_oEmkvK1poIck@z> Hi Balbir, On Sat, Oct 27, 2018 at 09:21:02PM +1100, Balbir Singh wrote: > On Wed, Oct 24, 2018 at 07:13:50PM -0700, Joel Fernandes wrote: > > On Wed, Oct 24, 2018 at 10:57:33PM +1100, Balbir Singh wrote: > > [...] > > > > > + pmd_t pmd; > > > > > + > > > > > + new_ptl = pmd_lockptr(mm, new_pmd); > > > > > > > > > Looks like this is largely inspired by move_huge_pmd(), I guess a lot of > > > the code applies, why not just reuse as much as possible? The same comments > > > w.r.t mmap_sem helping protect against lock order issues applies as well. > > > > I thought about this and when I looked into it, it seemed there are subtle > > differences that make such sharing not worth it (or not possible). > > > > Could you elaborate on them? The move_huge_page function is defined only for CONFIG_TRANSPARENT_HUGEPAGE so we cannot reuse it to begin with, since we have it disabled on our systems. I am not sure if it is a good idea to split that out and refactor it for reuse especially since our case is quite simple compared to huge pages. There are also a couple of subtle differences between the move_normal_pmd and the move_huge_pmd. Atleast 2 of them are: 1. We don't concern ourself with the PMD dirty bit, since the pages being moved are normal pages and at the soft-dirty bit accounting is at the PTE level, since we are not moving PTEs, we don't need to do that. 2. The locking is simpler as Kirill pointed, pmd_lock cannot fail however __pmd_trans_huge_lock can. I feel it is not super useful to refactor move_huge_pmd to support our case especially since move_normal_pmd is quite small, so IMHO the benefit of code reuse isn't there very much. Do let me know your thoughts and thanks for your interest in this. thanks, - Joel _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv