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[2003:cb:c74b:d000:3a9:de5c:9ae6:ccb3]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-37d7fa7a256sm4127405f8f.14.2024.10.16.04.54.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 16 Oct 2024 04:54:34 -0700 (PDT) Message-ID: Date: Wed, 16 Oct 2024 13:54:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 26/39] KVM: guest_memfd: Track faultability within a struct kvm_gmem_private To: Vishal Annapurve Cc: Ackerley Tng , Peter Xu , tabba@google.com, quic_eberman@quicinc.com, roypat@amazon.co.uk, jgg@nvidia.com, rientjes@google.com, fvdl@google.com, jthoughton@google.com, seanjc@google.com, pbonzini@redhat.com, zhiquan1.li@intel.com, fan.du@intel.com, jun.miao@intel.com, isaku.yamahata@intel.com, muchun.song@linux.dev, erdemaktas@google.com, qperret@google.com, jhubbard@nvidia.com, willy@infradead.org, shuah@kernel.org, brauner@kernel.org, bfoster@redhat.com, kent.overstreet@linux.dev, pvorel@suse.cz, rppt@kernel.org, richard.weiyang@gmail.com, anup@brainfault.org, haibo1.xu@intel.com, ajones@ventanamicro.com, vkuznets@redhat.com, maciej.wieczor-retman@intel.com, pgonda@google.com, oliver.upton@linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org References: <9abab5ad-98c0-48bb-b6be-59f2b3d3924a@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 71FF84001E X-Stat-Signature: k6ft91xou98xoai7tngkzoyap8srdx5e X-Rspam-User: X-HE-Tag: 1729079668-298412 X-HE-Meta: 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 pC2k7p18 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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 16.10.24 12:48, Vishal Annapurve wrote: > On Wed, Oct 16, 2024 at 2:20 PM David Hildenbrand wrote: >> >>>> I also don't know how you treat things like folio_test_hugetlb() on >>>> possible assumptions that the VMA must be a hugetlb vma. I'd confess I >>>> didn't yet check the rest of the patchset yet - reading a large series >>>> without a git tree is sometimes challenging to me. >>>> >>> >>> I'm thinking to basically never involve folio_test_hugetlb(), and the >>> VMAs used by guest_memfd will also never be a HugeTLB VMA. That's >>> because only the HugeTLB allocator is used, but by the time the folio is >>> mapped to userspace, it would have already have been split. After the >>> page is split, the folio loses its HugeTLB status. guest_memfd folios >>> will never be mapped to userspace while they still have a HugeTLB >>> status. >> >> We absolutely must convert these hugetlb folios to non-hugetlb folios. >> >> That is one of the reasons why I raised at LPC that we should focus on >> leaving hugetlb out of the picture and rather have a global pool, and >> the option to move folios from the global pool back and forth to hugetlb >> or to guest_memfd. >> >> How exactly that would look like is TBD. >> >> For the time being, I think we could add a "hack" to take hugetlb folios >> from hugetlb for our purposes, but we would absolutely have to convert >> them to non-hugetlb folios, especially when we split them to small >> folios and start using the mapcount. But it doesn't feel quite clean. > > As hugepage folios need to be split up in order to support backing > CoCo VMs with hugepages, I would assume any folio based hugepage > memory allocation will need to go through split/merge cycles through > the guest memfd lifetime. Yes, that's my understanding as well. > > Plan through next RFC series is to abstract out the hugetlb folio > management within guest_memfd so that any hugetlb specific logic is > cleanly separated out and allows guest memfd to allocate memory from > other hugepage allocators in the future. Yes, that must happen. As soon as a hugetlb folio would transition to guest_memfd, it must no longer be a hugetlb folio. > >> >> Simply starting with a separate global pool (e.g., boot-time allocation >> similar to as done by hugetlb, or CMA) might be cleaner, and a lot of >> stuff could be factored out from hugetlb code to achieve that. > > I am not sure if a separate global pool necessarily solves all the > issues here unless we come up with more concrete implementation > details. One of the concerns was the ability of implementing/retaining > HVO while transferring memory between the separate global pool and > hugetlb pool i.e. whether it can seamlessly serve all hugepage users > on the host. Likely should be doable. All we need is the generalized concept of a folio with HVO, and a way to move these folios between owners (e.g., global<->hugetlb, global<->guest_memfd). Factoring the HVO optimization out shouldn't be too crazy I believe. Famous last words :) > Another question could be whether the separate > pool/allocator simplifies the split/merge operations at runtime. The less hugetlb hacks we have to add, the better :) -- Cheers, David / dhildenb