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[2003:cb:c741:2200:2adc:9a8d:ae91:2e9f]) by smtp.gmail.com with ESMTPSA id e13-20020a05600c218d00b00412cfdc41f7sm439884wme.0.2024.03.19.02.54.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 19 Mar 2024 02:54:22 -0700 (PDT) Message-ID: <73f5f05b-2826-49ea-a3fd-3f1a5b14a223@redhat.com> Date: Tue, 19 Mar 2024 10:54:19 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: folio_mmapped To: Quentin Perret Cc: Sean Christopherson , Matthew Wilcox , Fuad Tabba , 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, viro@zeniv.linux.org.uk, 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: <755911e5-8d4a-4e24-89c7-a087a26ec5f6@redhat.com> <99a94a42-2781-4d48-8b8c-004e95db6bb5@redhat.com> <20240304132732963-0800.eberman@hu-eberman-lv.qualcomm.com> <4b0fd46a-cc4f-4cb7-9f6f-ce19a2d3064e@redhat.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: 9494C100008 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: 4zxjheadzjwrcraphzik6iobd6d1oath X-HE-Tag: 1710842068-588966 X-HE-Meta: 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 csQhgAgr ytutAFYFoZqjbEk+Cv8Rn+N+qYGJolyEDVbCVES33pRbGWu+2WlvfGxGEbNpF9BYsa4I2+omuEF4z8eTW8Poo5jRU3JpNrn5VApZ61n/+XryvwUzhtbhrtUgQoV6ju7jibLg2b9ThTL1GFLaA4iXV9HTOcKSO8wdQHU7lIFH562vWoPHdhO7rQa5+S/9BXMyu7uSmB/UCxwc2iM+6mE4fKaVYmBuW6FInwGx4yt1Wt6IEN5Tfw+0toI01hCdiEa6kFhAFN2azvSpFYNsKb2VRgNDMGhXK7BGutWmQ 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 19.03.24 10:47, Quentin Perret wrote: > On Monday 04 Mar 2024 at 22:58:49 (+0100), David Hildenbrand wrote: >> On 04.03.24 22:43, Elliot Berman wrote: >>> On Mon, Mar 04, 2024 at 09:17:05PM +0100, David Hildenbrand wrote: >>>> On 04.03.24 20:04, Sean Christopherson wrote: >>>>> On Mon, Mar 04, 2024, Quentin Perret wrote: >>>>>>> As discussed in the sub-thread, that might still be required. >>>>>>> >>>>>>> One could think about completely forbidding GUP on these mmap'ed >>>>>>> guest-memfds. But likely, there might be use cases in the future where you >>>>>>> want to use GUP on shared memory inside a guest_memfd. >>>>>>> >>>>>>> (the iouring example I gave might currently not work because >>>>>>> FOLL_PIN|FOLL_LONGTERM|FOLL_WRITE only works on shmem+hugetlb, and >>>>>>> guest_memfd will likely not be detected as shmem; 8ac268436e6d contains some >>>>>>> details) >>>>>> >>>>>> Perhaps it would be wise to start with GUP being forbidden if the >>>>>> current users do not need it (not sure if that is the case in Android, >>>>>> I'll check) ? We can always relax this constraint later when/if the >>>>>> use-cases arise, which is obviously much harder to do the other way >>>>>> around. >>>>> >>>>> +1000. At least on the KVM side, I would like to be as conservative as possible >>>>> when it comes to letting anything other than the guest access guest_memfd. >>>> >>>> So we'll have to do it similar to any occurrences of "secretmem" in gup.c. >>>> We'll have to see how to marry KVM guest_memfd with core-mm code similar to >>>> e.g., folio_is_secretmem(). >>>> >>>> IIRC, we might not be able to de-reference the actual mapping because it >>>> could get free concurrently ... >>>> >>>> That will then prohibit any kind of GUP access to these pages, including >>>> reading/writing for ptrace/debugging purposes, for core dumping purposes >>>> etc. But at least, you know that nobody was able to optain page references >>>> using GUP that might be used for reading/writing later. >>> >>> Do you have any concerns to add to enum mapping_flags, AS_NOGUP, and >>> replacing folio_is_secretmem() with a test of this bit instead of >>> comparing the a_ops? I think it scales better. >> >> The only concern I have are races, but let's look into the details: >> >> In GUP-fast, we can essentially race with unmap of folios, munmap() of VMAs >> etc. >> >> We had a similar discussion recently about possible races. It's documented >> in folio_fast_pin_allowed() regarding disabled IRQs and RCU grace periods. >> >> "inodes and thus their mappings are freed under RCU, which means the mapping >> cannot be freed beneath us and thus we can safely dereference it." >> >> So if we follow the same rules as folio_fast_pin_allowed(), we can >> de-reference folio->mapping, for example comparing mapping->a_ops. >> >> [folio_is_secretmem should better follow the same approach] > > Resurecting this discussion, I had discussions internally and as it > turns out Android makes extensive use of vhost/vsock when communicating > with guest VMs, which requires GUP. So, my bad, not supporting GUP for > the pKVM variant of guest_memfd is a bit of a non-starter, we'll need to > support it from the start. But again this should be a matter of 'simply' > having a dedicated KVM exit reason so hopefully it's not too bad. Likely you should look into ways to teach these interfaces that require GUP to consume fd+offset instead. Yes, there is no such thing as free lunch :P -- Cheers, David / dhildenb