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 67C03C04E84 for ; Thu, 16 May 2019 11:16:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3A930206BF for ; Thu, 16 May 2019 11:16:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1558005371; bh=wPq5dQpBho1HqAal08Xih0Vccmr15L0E+zjQKxqbDZg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=dNVzuRfLhdjuNo8BHjImTmOXdz13cPx7a/g8RPogRvnPemW4LagLO2DpHEHNQIilV ZWAWp/kTrbYXFXnB558eEGAgCuJRM6N9P/NxuM90qr+yaOsCLXUy6WdwKSEHK97WX4 LM4XgsqoB3TdPklt05zS+0Qg0TXd2RjlJbYyG0G0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727144AbfEPLQK (ORCPT ); Thu, 16 May 2019 07:16:10 -0400 Received: from mx2.suse.de ([195.135.220.15]:32922 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726537AbfEPLQJ (ORCPT ); Thu, 16 May 2019 07:16:09 -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 AD50EAEF7; Thu, 16 May 2019 11:16:07 +0000 (UTC) Date: Thu, 16 May 2019 13:16:07 +0200 From: Michal Hocko To: Anshuman Khandual Cc: Mark Rutland , 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: <20190516111607.GR16651@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: 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 16:36:12, Anshuman Khandual wrote: > On 05/16/2019 03:53 PM, 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. > > > > 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. > > > >> 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()? > > Still wondering how pfn_to_online_page() is applicable here. It validates > a given PFN and whether its online from sparse section mapping perspective > before giving it's struct page. IIUC it is used during a linear scanning > of a physical address range not for a page table walk. So how it can solve > the problem when a struct page which was used as an intermediate level page > table page gets released back to the buddy from another concurrent thread ? Well, my comment about pfn_to_online_page was more generic and it might not apply to this specific case. I meant to say that the code outside of the core MM shouldn't really care about the hotplug locking. -- Michal Hocko SUSE Labs