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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, 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 C7FD6C04AAF for ; Thu, 16 May 2019 11:05:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 990892070D for ; Thu, 16 May 2019 11:05:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1558004733; bh=O/sPEBR1Gl8B2HXyFlFvhbUhktWtu1ihThZCnciBYhQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=TiVzyw0QxBgJT1DMLOSoWUWRpWFlRDXsPZ2uoyBzIj1r5dvi4xuQAJishPjau26Hf MYC4NKrg260/hikFLv/7x1TX3qj107t8rB2cYjaBgKYmtTkgNqwSo609pnJyZbI+37 c+U5Ys4iP6OIvwENjP+JHB7MxnAXNnY7Twqboz9k= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726975AbfEPLFc (ORCPT ); Thu, 16 May 2019 07:05:32 -0400 Received: from mx2.suse.de ([195.135.220.15]:59196 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726597AbfEPLFc (ORCPT ); Thu, 16 May 2019 07:05:32 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 24A6FAEF7; Thu, 16 May 2019 11:05:30 +0000 (UTC) Date: Thu, 16 May 2019 13:05:29 +0200 From: Michal Hocko To: Mark Rutland Cc: Anshuman Khandual , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org, catalin.marinas@arm.com, will.deacon@arm.com, mgorman@techsingularity.net, james.morse@arm.com, robin.murphy@arm.com, cpandya@codeaurora.org, arunks@codeaurora.org, dan.j.williams@intel.com, osalvador@suse.de, david@redhat.com, cai@lca.pw, logang@deltatee.com, ira.weiny@intel.com Subject: Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump Message-ID: <20190516110529.GQ16651@dhcp22.suse.cz> References: <1557824407-19092-1-git-send-email-anshuman.khandual@arm.com> <1557824407-19092-3-git-send-email-anshuman.khandual@arm.com> <20190515165847.GH16651@dhcp22.suse.cz> <20190516102354.GB40960@lakrids.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190516102354.GB40960@lakrids.cambridge.arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 16-05-19 11:23:54, Mark Rutland wrote: > Hi Michal, > > On Wed, May 15, 2019 at 06:58:47PM +0200, Michal Hocko wrote: > > On Tue 14-05-19 14:30:05, Anshuman Khandual wrote: > > > The arm64 pagetable dump code can race with concurrent modification of the > > > kernel page tables. When a leaf entries are modified concurrently, the dump > > > code may log stale or inconsistent information for a VA range, but this is > > > otherwise not harmful. > > > > > > When intermediate levels of table are freed, the dump code will continue to > > > use memory which has been freed and potentially reallocated for another > > > purpose. In such cases, the dump code may dereference bogus addressses, > > > leading to a number of potential problems. > > > > > > Intermediate levels of table may by freed during memory hot-remove, or when > > > installing a huge mapping in the vmalloc region. To avoid racing with these > > > cases, take the memory hotplug lock when walking the kernel page table. > > > > Why is this a problem only on arm64 > > It looks like it's not -- I think we're just the first to realise this. > > AFAICT x86's debugfs ptdump has the same issue if run conccurently with > memory hot remove. If 32-bit arm supported hot-remove, its ptdump code > would have the same issue. > > > and why do we even care for debugfs? Does anybody rely on this thing > > to be reliable? Do we even need it? Who is using the file? > > The debugfs part is used intermittently by a few people working on the > arm64 kernel page tables. We use that both to sanity-check that kernel > page tables are created/updated correctly after changes to the arm64 mmu > code, and also to debug issues if/when we encounter issues that appear > to be the result of kernel page table corruption. OK, I see. Thanks for the clarification. > So while it's rare to need it, it's really useful to have when we do > need it, and I'd rather not remove it. I'd also rather that it didn't > have latent issues where we can accidentally crash the kernel when using > it, which is what this patch is addressing. While I agree, do we rather want to document that you shouldn't be using the debugging tool while the hotplug is ongoing because you might get a garbage or crash the kernel in the worst case? In other words is the absolute correctness worth the additional maint. burden wrt. to future hotplug changes? > > I am asking because I would really love to make mem hotplug locking less > > scattered outside of the core MM than more. Most users simply shouldn't > > care. Pfn walkers should rely on pfn_to_online_page. > > I'm not sure if that would help us here; IIUC pfn_to_online_page() alone > doesn't ensure that the page remains online. Is there a way to achieve > that other than get_online_mems()? You have to pin the page to make sure the hotplug is not going to offline it. > The big problem for the ptdump code is when tables are freed, since the > pages can be reused elsewhere (or hot-removed), causing the ptdump code > to explode. Yes, I see the danger. I am just wondering whether living with that is reasonable considering this is a debugfs code. -- Michal Hocko SUSE Labs