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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 7BE94C5DF60 for ; Wed, 6 Nov 2019 00:12:47 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (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 04796214D8 for ; Wed, 6 Nov 2019 00:12:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 04796214D8 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 4776Rx2RgSzF5Gg for ; Wed, 6 Nov 2019 11:12:45 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=intel.com (client-ip=192.55.52.93; helo=mga11.intel.com; envelope-from=sean.j.christopherson@intel.com; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=intel.com Received: from mga11.intel.com (mga11.intel.com [192.55.52.93]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4775mk44m9zF4QM for ; Wed, 6 Nov 2019 10:42:14 +1100 (AEDT) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by fmsmga102.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 05 Nov 2019 15:42:10 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.68,271,1569308400"; d="scan'208";a="200541599" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.41]) by fmsmga008.fm.intel.com with ESMTP; 05 Nov 2019 15:42:08 -0800 Date: Tue, 5 Nov 2019 15:42:08 -0800 From: Sean Christopherson To: Dan Williams Subject: Re: [PATCH v1 03/10] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes Message-ID: <20191105234208.GH23297@linux.intel.com> References: <20191024120938.11237-4-david@redhat.com>

<01adb4cb-6092-638c-0bab-e61322be7cf5@redhat.com> <613f3606-748b-0e56-a3ad-1efaffa1a67b@redhat.com> <20191105160000.GC8128@linux.intel.com>

<20191105231316.GE23297@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) X-Mailman-Approved-At: Wed, 06 Nov 2019 11:05:37 +1100 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-hyperv@vger.kernel.org, Michal Hocko , Radim =?utf-8?B?S3LEjW3DocWZ?= , KVM list , David Hildenbrand , KarimAllah Ahmed , Dave Hansen , Alexander Duyck , Michal Hocko , Linux MM , Pavel Tatashin , Paul Mackerras , "H. Peter Anvin" , Wanpeng Li , Alexander Duyck , "K. Y. Srinivasan" , Thomas Gleixner , Kees Cook , devel@driverdev.osuosl.org, Stefano Stabellini , Stephen Hemminger , "Aneesh Kumar K.V" , Joerg Roedel , X86 ML , YueHaibing , "Matthew Wilcox \(Oracle\)" , Mike Rapoport , Peter Zijlstra , Ingo Molnar , Vlastimil Babka , Anthony Yznaga , Oscar Salvador , "Isaac J. Manjarres" , Matt Sickler , Juergen Gross , Anshuman Khandual , Haiyang Zhang , Sasha Levin , kvm-ppc@vger.kernel.org, Qian Cai , Alex Williamson , Mike Rapoport , Borislav Petkov , Nicholas Piggin , Andy Lutomirski , xen-devel , Boris Ostrovsky , Vitaly Kuznetsov , Allison Randal , Jim Mattson , Mel Gorman , Adam Borowski , Cornelia Huck , Pavel Tatashin , Linux Kernel Mailing List , Johannes Weiner , Paolo Bonzini , Andrew Morton , linuxppc-dev Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Nov 05, 2019 at 03:30:00PM -0800, Dan Williams wrote: > On Tue, Nov 5, 2019 at 3:13 PM Sean Christopherson > wrote: > > > > On Tue, Nov 05, 2019 at 03:02:40PM -0800, Dan Williams wrote: > > > On Tue, Nov 5, 2019 at 12:31 PM David Hildenbrand wrote: > > > > > The scarier code (for me) is transparent_hugepage_adjust() and > > > > > kvm_mmu_zap_collapsible_spte(), as I don't at all understand the > > > > > interaction between THP and _PAGE_DEVMAP. > > > > > > > > The x86 KVM MMU code is one of the ugliest code I know (sorry, but it > > > > had to be said :/ ). Luckily, this should be independent of the > > > > PG_reserved thingy AFAIKs. > > > > > > Both transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte() > > > are honoring kvm_is_reserved_pfn(), so again I'm missing where the > > > page count gets mismanaged and leads to the reported hang. > > > > When mapping pages into the guest, KVM gets the page via gup(), which > > increments the page count for ZONE_DEVICE pages. But KVM puts the page > > using kvm_release_pfn_clean(), which skips put_page() if PageReserved() > > and so never puts its reference to ZONE_DEVICE pages. > > Oh, yeah, that's busted. > > > My transparent_hugepage_adjust() and kvm_mmu_zap_collapsible_spte() > > comments were for a post-patch/series scenario wheren PageReserved() is > > no longer true for ZONE_DEVICE pages. > > Ah, ok, for that David is preserving kvm_is_reserved_pfn() returning > true for ZONE_DEVICE because pfn_to_online_page() will fail for > ZONE_DEVICE. But David's proposed fix for the above refcount bug is to omit the patch so that KVM no longer treats ZONE_DEVICE pages as reserved. That seems like the right thing to do, including for thp_adjust(), e.g. it would naturally let KVM use 2mb pages for the guest when a ZONE_DEVICE page is mapped with a huge page (2mb or above) in the host. The only hiccup is figuring out how to correctly transfer the reference.