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[2003:cb:c733:f100:75e7:a0a4:9ac2:1abb]) by smtp.gmail.com with ESMTPSA id i4-20020a05600c354400b004101f27737asm18975536wmq.29.2024.03.04.12.22.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 04 Mar 2024 12:22:47 -0800 (PST) Message-ID: <2773d2e1-9d73-4d38-97f1-4d90c17b2e26@redhat.com> Date: Mon, 4 Mar 2024 21:22:44 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: folio_mmapped To: Quentin Perret Cc: Fuad Tabba , Matthew Wilcox , kvm@vger.kernel.org, kvmarm@lists.linux.dev, pbonzini@redhat.com, chenhuacai@kernel.org, mpe@ellerman.id.au, anup@brainfault.org, paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, seanjc@google.com, brauner@kernel.org, akpm@linux-foundation.org, xiaoyao.li@intel.com, yilun.xu@intel.com, chao.p.peng@linux.intel.com, jarkko@kernel.org, amoorthy@google.com, dmatlack@google.com, yu.c.zhang@linux.intel.com, isaku.yamahata@intel.com, mic@digikod.net, vbabka@suse.cz, vannapurve@google.com, ackerleytng@google.com, mail@maciej.szmigiero.name, michael.roth@amd.com, wei.w.wang@intel.com, liam.merwick@oracle.com, isaku.yamahata@gmail.com, kirill.shutemov@linux.intel.com, suzuki.poulose@arm.com, steven.price@arm.com, quic_mnalajal@quicinc.com, quic_tsoni@quicinc.com, quic_svaddagi@quicinc.com, quic_cvanscha@quicinc.com, quic_pderrin@quicinc.com, quic_pheragu@quicinc.com, catalin.marinas@arm.com, james.morse@arm.com, yuzenghui@huawei.com, oliver.upton@linux.dev, maz@kernel.org, will@kernel.org, keirf@google.com, linux-mm@kvack.org References: <925f8f5d-c356-4c20-a6a5-dd7efde5ee86@redhat.com> <755911e5-8d4a-4e24-89c7-a087a26ec5f6@redhat.com> <99a94a42-2781-4d48-8b8c-004e95db6bb5@redhat.com> <20240229114526893-0800.eberman@hu-eberman-lv.qualcomm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 60681C0007 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: mdr7mgt1zte7i598w54ynrmjh4p44gma X-HE-Tag: 1709583775-77043 X-HE-Meta: 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 djveNCnj 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 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: List-Subscribe: List-Unsubscribe: On 04.03.24 13:53, Quentin Perret wrote: > On Friday 01 Mar 2024 at 12:16:54 (+0100), David Hildenbrand wrote: >>>> I don't think that we can assume that only a single VMA covers a page. >>>> >>>>> But of course, no rmap walk is always better. >>>> >>>> We've been thinking some more about how to handle the case where the >>>> host userspace has a mapping of a page that later becomes private. >>>> >>>> One idea is to refuse to run the guest (i.e., exit vcpu_run() to back >>>> to the host with a meaningful exit reason) until the host unmaps that >>>> page, and check for the refcount to the page as you mentioned earlier. >>>> This is essentially what the RFC I sent does (minus the bugs :) ) . >>>> >>>> The other idea is to use the rmap walk as you suggested to zap that >>>> page. If the host tries to access that page again, it would get a >>>> SIGBUS on the fault. This has the advantage that, as you'd mentioned, >>>> the host doesn't need to constantly mmap() and munmap() pages. It >>>> could potentially be optimised further as suggested if we have a >>>> cooperating VMM that would issue a MADV_DONTNEED or something like >>>> that, but that's just an optimisation and we would still need to have >>>> the option of the rmap walk. However, I was wondering how practical >>>> this idea would be if more than a single VMA covers a page? >>>> >>> >>> Agree with all your points here. I changed Gunyah's implementation to do >>> the unmap instead of erroring out. I didn't observe a significant >>> performance difference. However, doing unmap might be a little faster >>> because we can check folio_mapped() before doing the rmap walk. When >>> erroring out at mmap() level, we always have to do the walk. >> >> Right. On the mmap() level you won't really have to walk page tables, as the >> the munmap() already zapped the page and removed the "problematic" VMA. >> >> Likely, you really want to avoid repeatedly calling mmap()+munmap() just to >> access shared memory; but that's just my best guess about your user space >> app :) > > Ack, and expecting userspace to munmap the pages whenever we hit a valid > mapping in userspace page-tables in the KVM faults path makes for a > somewhat unusual interface IMO. Userspace can munmap, mmap again, and if > it doesn't touch the pages, it can proceed to run the guest just fine, > is that the expectation? If so, it feels like we're 'leaking' internal It would be weird, and I would not suggest that. It's either (1) you can leave it mmap'ed, but any access to private memory will SIGBUS. The kernel will try zapping pages inside a VMA itself. (2) you cannot leave it mmap'ed. In order to convert shared -> private, you have to munmap. mmap will fail if it would cover a currently-private page. So for (1) you could mmap once in user space and be done with it. For (2) you would have to mmap+munmap when accessing shared memory. > kernel state somehow. The kernel is normally well within its rights to > zap userspace mappings if it wants to e.g. swap. (Obviously mlock is a > weird case, but even in that case, IIRC the kernel still has a certain > amount of flexibility and can use compaction and friends). Similarly, > it should be well within its right to proactively create them. How > would this scheme work if, 10 years from now, something like > Speculative Page Faults makes it into the kernel in a different form? At least with (2), speculative page faults would never generate a SIGBUS. Just like HW must not generate a fault on speculative access. > > Not requiring to userspace to unmap makes the userspace interface a lot > simpler I think -- once a protected guest starts, you better not touch > its memory if it's not been shared back or you'll get slapped on the > wrist. Whether or not those pages have been accessed beforehand for > example is irrelevant. Yes. So the theory :) -- Cheers, David / dhildenb