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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CB1B9C43334 for ; Wed, 15 Jun 2022 09:21:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243260AbiFOJVd (ORCPT ); Wed, 15 Jun 2022 05:21:33 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33588 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239736AbiFOJVd (ORCPT ); Wed, 15 Jun 2022 05:21:33 -0400 Received: from mga07.intel.com (mga07.intel.com [134.134.136.100]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DEFA81D338; Wed, 15 Jun 2022 02:21:31 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1655284892; x=1686820892; h=date:from:to:cc:subject:message-id:reply-to:references: mime-version:in-reply-to; bh=35w+G+fbWucz8txfjhA1uFhTpuj87R9d/qHcnOEmPeM=; b=CB+v7hMtlCwATy66WC+JXDAigU3Duu1OHQaxlOXRB+Se+IXtUYWuiTw3 cCn+uBJ5EpHDvUSedqpLVd82Wt1pXsYU3vIUnLC7iMgxk6LDRU20tzONE FBVwS3M7xCtCUQTYJLkDlQa5owb7jTQAP8d17CwsWqp7uHCqocVslTvdy IYuN9Hf3FQyAmOTPQXfPYgZdOil0fU7P6YLvDmGeEDdXs8D3KLN94ZSGS CPgj1DyOARMY9eC6HfjG5M8naa39SvcZB7P7d+etiL+uJM4EklkXNQpmf sGib3sUeBImtc6RnxBWyyw3uXjPy//yyb4vpzKRW6bUK9E2Vq/wmy0Xgd Q==; X-IronPort-AV: E=McAfee;i="6400,9594,10378"; a="342856863" X-IronPort-AV: E=Sophos;i="5.91,300,1647327600"; d="scan'208";a="342856863" Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 15 Jun 2022 02:21:31 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.91,300,1647327600"; d="scan'208";a="583119186" Received: from chaop.bj.intel.com (HELO localhost) ([10.240.192.101]) by orsmga007.jf.intel.com with ESMTP; 15 Jun 2022 02:21:21 -0700 Date: Wed, 15 Jun 2022 17:17:59 +0800 From: Chao Peng To: Andy Lutomirski Cc: Sean Christopherson , Vishal Annapurve , Marc Orr , kvm list , LKML , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-doc@vger.kernel.org, qemu-devel@nongnu.org, Paolo Bonzini , Jonathan Corbet , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86 , "H . Peter Anvin" , Hugh Dickins , Jeff Layton , "J . Bruce Fields" , Andrew Morton , Mike Rapoport , Steven Price , "Maciej S . Szmigiero" , Vlastimil Babka , Yu Zhang , "Kirill A . Shutemov" , Jun Nakajima , Dave Hansen , Andi Kleen , David Hildenbrand , aarcange@redhat.com, ddutile@redhat.com, dhildenb@redhat.com, Quentin Perret , Michael Roth , mhocko@suse.com Subject: Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory Message-ID: <20220615091759.GB1823790@chaop.bj.intel.com> Reply-To: Chao Peng References: <20220607065749.GA1513445@chaop.bj.intel.com> <20220608021820.GA1548172@chaop.bj.intel.com> <20220614072800.GB1783435@chaop.bj.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Tue, Jun 14, 2022 at 01:59:41PM -0700, Andy Lutomirski wrote: > On Tue, Jun 14, 2022 at 12:09 PM Sean Christopherson wrote: > > > > On Tue, Jun 14, 2022, Andy Lutomirski wrote: > > > On Tue, Jun 14, 2022 at 12:32 AM Chao Peng wrote: > > > > > > > > On Thu, Jun 09, 2022 at 08:29:06PM +0000, Sean Christopherson wrote: > > > > > On Wed, Jun 08, 2022, Vishal Annapurve wrote: > > > > > > > > > > One argument is that userspace can simply rely on cgroups to detect misbehaving > > > > > guests, but (a) those types of OOMs will be a nightmare to debug and (b) an OOM > > > > > kill from the host is typically considered a _host_ issue and will be treated as > > > > > a missed SLO. > > > > > > > > > > An idea for handling this in the kernel without too much complexity would be to > > > > > add F_SEAL_FAULT_ALLOCATIONS (terrible name) that would prevent page faults from > > > > > allocating pages, i.e. holes can only be filled by an explicit fallocate(). Minor > > > > > faults, e.g. due to NUMA balancing stupidity, and major faults due to swap would > > > > > still work, but writes to previously unreserved/unallocated memory would get a > > > > > SIGSEGV on something it has mapped. That would allow the userspace VMM to prevent > > > > > unintentional allocations without having to coordinate unmapping/remapping across > > > > > multiple processes. > > > > > > > > Since this is mainly for shared memory and the motivation is catching > > > > misbehaved access, can we use mprotect(PROT_NONE) for this? We can mark > > > > those range backed by private fd as PROT_NONE during the conversion so > > > > subsequence misbehaved accesses will be blocked instead of causing double > > > > allocation silently. > > > > PROT_NONE, a.k.a. mprotect(), has the same vma downsides as munmap(). Yes, right. > > > > > This patch series is fairly close to implementing a rather more > > > efficient solution. I'm not familiar enough with hypervisor userspace > > > to really know if this would work, but: > > > > > > What if shared guest memory could also be file-backed, either in the > > > same fd or with a second fd covering the shared portion of a memslot? > > > This would allow changes to the backing store (punching holes, etc) to > > > be some without mmap_lock or host-userspace TLB flushes? Depending on > > > what the guest is doing with its shared memory, userspace might need > > > the memory mapped or it might not. > > > > That's what I'm angling for with the F_SEAL_FAULT_ALLOCATIONS idea. The issue, > > unless I'm misreading code, is that punching a hole in the shared memory backing > > store doesn't prevent reallocating that hole on fault, i.e. a helper process that > > keeps a valid mapping of guest shared memory can silently fill the hole. > > > > What we're hoping to achieve is a way to prevent allocating memory without a very > > explicit action from userspace, e.g. fallocate(). > > Ah, I misunderstood. I thought your goal was to mmap it and prevent > page faults from allocating. I think we still need the mmap, but want to prevent allocating when userspace touches previously mmaped area that has never filled the page. I don't have clear answer if other operations like read/write should be also prevented (probably yes). And only after an explicit fallocate() to allocate the page these operations would act normally. > > It is indeed the case (and has been since before quite a few of us > were born) that a hole in a sparse file is logically just a bunch of > zeros. A way to make a file for which a hole is an actual hole seems > like it would solve this problem nicely. It could also be solved more > specifically for KVM by making sure that the private/shared mode that > userspace programs is strict enough to prevent accidental allocations > -- if a GPA is definitively private, shared, neither, or (potentially, > on TDX only) both, then a page that *isn't* shared will never be > accidentally allocated by KVM. KVM is clever enough to not allocate since it knows a GPA is shared or not. This case it's the host userspace that can cause the allocating and is too complex to check on every access from guest. > If the shared backing is not mmapped, > it also won't be accidentally allocated by host userspace on a stray > or careless write. As said above, mmap is still prefered, otherwise too many changes are needed for usespace VMM. Thanks, Chao > > > --Andy