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=-1.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 1A9E2C04AAF for ; Thu, 16 May 2019 11:06:27 +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 E673D2082E for ; Thu, 16 May 2019 11:06:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="JK9sZHwc" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E673D2082E Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date: Message-ID:From:References:To:Subject:Reply-To:Content-ID:Content-Description :Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=XFc3jrHJPDlsj2IRCcBTITJ/zpQPwHGSblc2HaECSO0=; b=JK9sZHwcMq0xHu vFORpqYzNcXTIjU7JrDeKny95MzsBnh8rTuTRpYhVVNvsYpHHECmkK9jKOiml+vr3CkNZKSJsZb36 rIRu9nUMPdFPFcTY/CWP8kxTBvowBwIJzxjpnpQK4+1D4Q6MLnrrap64ZR38J6jqzfVfTXc67IE9p M7O8k7ZsI8s53RQ6NAqHMKr1sYz/ldaSl18c2F+i1yXGnAsCoPKXkFfdqu7vwCFtnYL9wmqiuQ9BX a+PIvYhEOCbbcxvd21hZ5OpfLwgMbDxnp35zVTYH/I3Gae+rSZC7iJUZhDQg2af3rfaBZpAjy/pW4 cEItTLLRWnGkUcgCsw6w==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1hREDL-0003pq-BN; Thu, 16 May 2019 11:06:19 +0000 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70] helo=foss.arm.com) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1hREDI-0003p6-3z for linux-arm-kernel@lists.infradead.org; Thu, 16 May 2019 11:06:17 +0000 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 D9E1419BF; Thu, 16 May 2019 04:06:12 -0700 (PDT) Received: from [10.163.1.137] (unknown [10.163.1.137]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id AF0023F703; Thu, 16 May 2019 04:06:03 -0700 (PDT) Subject: Re: [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump To: Mark Rutland , Michal Hocko 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> From: Anshuman Khandual Message-ID: Date: Thu, 16 May 2019 16:36:12 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20190516102354.GB40960@lakrids.cambridge.arm.com> Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190516_040616_167589_E9CBEC48 X-CRM114-Status: GOOD ( 27.52 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: cai@lca.pw, ira.weiny@intel.com, david@redhat.com, catalin.marinas@arm.com, will.deacon@arm.com, linux-kernel@vger.kernel.org, logang@deltatee.com, james.morse@arm.com, cpandya@codeaurora.org, arunks@codeaurora.org, akpm@linux-foundation.org, osalvador@suse.de, mgorman@techsingularity.net, dan.j.williams@intel.com, linux-arm-kernel@lists.infradead.org, robin.murphy@arm.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 ? _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel