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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4CE23C43334 for ; Wed, 15 Jun 2022 09:21:39 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id D97DD6B0071; Wed, 15 Jun 2022 05:21:38 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D480A6B0072; Wed, 15 Jun 2022 05:21:38 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id BE92F6B0073; Wed, 15 Jun 2022 05:21:38 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id AFF806B0071 for ; Wed, 15 Jun 2022 05:21:38 -0400 (EDT) Received: from smtpin05.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay02.hostedemail.com (Postfix) with ESMTP id CB97434CF5 for ; Wed, 15 Jun 2022 09:21:37 +0000 (UTC) X-FDA: 79579927434.05.3E20C4D Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by imf18.hostedemail.com (Postfix) with ESMTP id 643FF1C007E for ; Wed, 15 Jun 2022 09:21:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1655284894; x=1686820894; h=date:from:to:cc:subject:message-id:reply-to:references: mime-version:in-reply-to; bh=35w+G+fbWucz8txfjhA1uFhTpuj87R9d/qHcnOEmPeM=; b=StG6kbU5TMNQalonFTrw8DbalhRZx8bA5NBZBqokk6Ii1xIQQvjvoY21 sTyiejGIWfRMybL2JWxdhZBqrfvAyl2REa14tulRwNBpPIhRTNCf1FJvt gGTZfeLwRF6rDd9k4/Jd6ADmto613jGvZra4Ytiv1cQlnbP/jrsrccO64 ViIGuOQ8duNgtEdp7jeeHjnnXLtXIsCdIzAI7voE3KKNUExtC/BHj2R5K HIwHyzf2kZHBDHAFguiAeh5+7m6qUjliBUMVpxZHqvMEVs4TD9NmEV3Zg AGrilVTOXWw2MGq10f4ZT3ZY/yO4U9wXQDBn+0vNSuIRBm4cLdrZWxDPj Q==; X-IronPort-AV: E=McAfee;i="6400,9594,10378"; a="277686798" X-IronPort-AV: E=Sophos;i="5.91,300,1647327600"; d="scan'208";a="277686798" Received: from orsmga007.jf.intel.com ([10.7.209.58]) by fmsmga104.fm.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: ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1655284895; h=from:from:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=GP2FJXp8y0y8/2gINhhkEBATQ3exVTO6sDNFFdr8j6U=; b=AMRSA7XxNncrBTxhKS25TcdO/Oxqd42P440kY21AH+ljDm8j/Pa+Tzz5NaGK5nvUY5eDmf IholQrcs4oINY1zOGTCg2Ze0hHsEdVkvPqE/JzgLjh+TT6Kfqxsj2zCtPHKd3yYM3fJACI nRPMtZj+J5XDwQuPnbtaum90CL0TqXQ= ARC-Authentication-Results: i=1; imf18.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=StG6kbU5; spf=none (imf18.hostedemail.com: domain of chao.p.peng@linux.intel.com has no SPF policy when checking 192.55.52.120) smtp.mailfrom=chao.p.peng@linux.intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1655284895; a=rsa-sha256; cv=none; b=bmKaf1aXiKtlBc5Ab+e/xCDPaDX98tkS35d0i4H9o1KG4yswjeCARd7n5Z086bLUx3hMhK 0QBv1+NmJslzuBRFkjJlYm/k1f7D4v3P4L98rtUxpLz2Nu82Htlr8SsNgsHcjC2XJrOWyN ebjbObeDYNsy9a17u6FwqR57aOQtMds= X-Stat-Signature: asfaf8gigrpuqi3c6k3ahqpmuzp4993s X-Rspamd-Queue-Id: 643FF1C007E X-Rspamd-Server: rspam11 X-Rspam-User: Authentication-Results: imf18.hostedemail.com; dkim=pass header.d=intel.com header.s=Intel header.b=StG6kbU5; spf=none (imf18.hostedemail.com: domain of chao.p.peng@linux.intel.com has no SPF policy when checking 192.55.52.120) smtp.mailfrom=chao.p.peng@linux.intel.com; dmarc=pass (policy=none) header.from=intel.com X-HE-Tag: 1655284894-182627 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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