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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id s8-20020a170902a50800b00161ac982b9esm7590467plq.185.2022.06.14.12.08.58 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Jun 2022 12:08:58 -0700 (PDT) Date: Tue, 14 Jun 2022 19:08:55 +0000 From: Sean Christopherson To: Andy Lutomirski Cc: Chao Peng , 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: References: <20220519153713.819591-1-chao.p.peng@linux.intel.com> <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-kernel@vger.kernel.org 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(). > 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().