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[2003:cb:c73f:8500:f83c:3602:5300:88af]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4282bb6403bsm246406305e9.35.2024.08.06.06.43.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 06 Aug 2024 06:43:29 -0700 (PDT) Message-ID: <883a0f0d-7342-479e-aa3c-13deb7e99338@redhat.com> Date: Tue, 6 Aug 2024 15:43:24 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 00/10] Introduce guestmemfs: persistent in-memory filesystem To: "Gowans, James" , "jack@suse.cz" , "muchun.song@linux.dev" Cc: "kvm@vger.kernel.org" , "rppt@kernel.org" , "brauner@kernel.org" , "Graf (AWS), Alexander" , "anthony.yznaga@oracle.com" , "steven.sistare@oracle.com" , "akpm@linux-foundation.org" , "linux-kernel@vger.kernel.org" , "Durrant, Paul" , "seanjc@google.com" , "pbonzini@redhat.com" , "linux-mm@kvack.org" , "Woodhouse, David" , "Saenz Julienne, Nicolas" , "viro@zeniv.linux.org.uk" , "nh-open-source@amazon.com" , "linux-fsdevel@vger.kernel.org" , "jgg@ziepe.ca" References: <20240805093245.889357-1-jgowans@amazon.com> <20240805200151.oja474ju4i32y5bj@quack3> <9802ddc299c72b189487fd56668de65a84f7d94b.camel@amazon.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: 112E1140018 X-Stat-Signature: ke1o3mor6ri8in3cbf3eq5fmo6znx9nf X-Rspamd-Server: rspam09 X-Rspam-User: X-HE-Tag: 1722951813-385244 X-HE-Meta: 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 i5KVY1wI sQKQgnvY7Nzf9Z+ZEQnkdYorjZ+SvlNjyOq4Me5ZN3wPJXCZezWTAkLK8mlHJ5F7Q9l6kLYVfI0rrVhtPgsz7P/LAdODxGOJ9Kh7Qf80/mSfWwHbfK44HWEy3FuBiRsqA+rePKh5/0P8xDVGM9yp05rQNYhVer/GkbC5f4xBO8ycuXYClJsQbrIDy1KIl0AKD4ITbUbRoVkp5rTwaXaVtN5ggBJs0IXL8sc3COd4mepCUBqF/dN1Gwe26+486dmJEfucNDcLaxeDfTnES87hP09H4wXhHcWiU2pa8nmw+HkV0OtEh+4039oULtvojMFMX6VzHFmOU7qP3w04H9V8VG9fpeSFHCLXYHBkfURXp0wF/5eza3Jx7NK0+cKDQynimHiKD45GXyQrZgfNd+7rvka9jb09jYNsDX1Kv 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: > 1. Secret hiding: with guestmemfs all of the memory is out of the kernel > direct map as an additional defence mechanism. This means no > read()/write() syscalls to guestmemfs files, and no IO to it. The only > way to access it is to mmap the file. There are people interested into similar things for guest_memfd. > > 2. No struct page overhead: the intended use case is for systems whose > sole job is to be a hypervisor, typically for large (multi-GiB) VMs, so > the majority of system RAM would be donated to this fs. We definitely > don't want 4 KiB struct pages here as it would be a significant > overhead. That's why guestmemfs carves the memory out in early boot and > sets memblock flags to avoid struct page allocation. I don't know if > hugetlbfs does anything fancy to avoid allocating PTE-level struct pages > for its memory? Sure, it's called HVO and can optimize out a significant portion of the vmemmap. > > 3. guest_memfd interface: For confidential computing use-cases we need > to provide a guest_memfd style interface so that these FDs can be used > as a guest_memfd file in KVM memslots. Would there be interest in > extending hugetlbfs to also support a guest_memfd style interface? > "Extending hugetlbfs" sounds wrong; hugetlbfs is a blast from the past and not something people are particularly keen to extend for such use cases. :) Instead, as Jason said, we're looking into letting guest_memfd own and manage large chunks of contiguous memory. > 4. Metadata designed for persistence: guestmemfs will need to keep > simple internal metadata data structures (limited allocations, limited > fragmentation) so that pages can easily and efficiently be marked as > persistent via KHO. Something like slab allocations would probably be a > no-go as then we'd need to persist and reconstruct the slab allocator. I > don't know how hugetlbfs structures its fs metadata but I'm guessing it > uses the slab and does lots of small allocations so trying to retrofit > persistence via KHO to it may be challenging. > > 5. Integration with persistent IOMMU mappings: to keep DMA running > across kexec, iommufd needs to know that the backing memory for an IOAS > is persistent too. The idea is to do some DMA pinning of persistent > files, which would require iommufd/guestmemfs integration - would we > want to add this to hugetlbfs? > > 6. Virtualisation-specific APIs: starting to get a bit esoteric here, > but use-cases like being able to carve out specific chunks of memory > from a running VM and turn it into memory for another side car VM, or > doing post-copy LM via DMA by mapping memory into the IOMMU but taking > page faults on the CPU. This may require virtualisation-specific ioctls > on the files which wouldn't be generally applicable to hugetlbfs. > > 7. NUMA control: a requirement is to always have correct NUMA affinity. > While currently not implemented the idea is to extend the guestmemfs > allocation to support specifying allocation sizes from each NUMA node at > early boot, and then having multiple mount points, one per NUMA node (or > something like that...). Unclear if this is something hugetlbfs would > want. > > There are probably more potential issues, but those are the ones that > come to mind... That being said, if hugetlbfs maintainers are interested > in going in this direction then we can definitely look at enhancing > hugetlbfs. > > I think there are two types of problems: "Would hugetlbfs want this > functionality?" - that's the majority. An a few are "This would be hard > with hugetlbfs!" - persistence probably falls into this category. I'm much rather asking myself if you should instead teach/extend the guest_memfd concept by some of what you propose here. At least "guest_memfd" sounds a lot like the "anonymous fd" based variant of guestmemfs ;) Like we have hugetlbfs and memfd with hugetlb pages. -- Cheers, David / dhildenb