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[2003:cb:c742:a200:af8e:d144:6284:4f93]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-37d7fc2e305sm3703197f8f.98.2024.10.16.01.50.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 16 Oct 2024 01:50:13 -0700 (PDT) Message-ID: <9abab5ad-98c0-48bb-b6be-59f2b3d3924a@redhat.com> Date: Wed, 16 Oct 2024 10:50:12 +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: Ackerley Tng , Peter Xu Cc: 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, vannapurve@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: From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: yxrbouwaj7srsxwi7z1jh9upsbdis7ok X-Rspamd-Queue-Id: B17F0C001A X-Rspamd-Server: rspam11 X-HE-Tag: 1729068607-515806 X-HE-Meta: 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 6Phj7dL6 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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: >> 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. 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. -- Cheers, David / dhildenb