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Wed, 01 Nov 2023 06:41:46 -0700 (PDT) Date: Wed, 1 Nov 2023 06:41:45 -0700 In-Reply-To: <92ba7ddd-2bc8-4a8d-bd67-d6614b21914f@intel.com> Mime-Version: 1.0 References: <20231027182217.3615211-1-seanjc@google.com> <20231027182217.3615211-18-seanjc@google.com> <7c0844d8-6f97-4904-a140-abeabeb552c1@intel.com> <92ba7ddd-2bc8-4a8d-bd67-d6614b21914f@intel.com> Message-ID: Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for dedicated guest memory From: Sean Christopherson To: Xiaoyao Li Cc: Paolo Bonzini , Marc Zyngier , Oliver Upton , Huacai Chen , Michael Ellerman , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexander Viro , Christian Brauner , "Matthew Wilcox (Oracle)" , Andrew Morton , kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Xu Yilun , Chao Peng , Fuad Tabba , Jarkko Sakkinen , Anish Moorthy , David Matlack , Yu Zhang , Isaku Yamahata , "=?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?=" , Vlastimil Babka , Vishal Annapurve , Ackerley Tng , Maciej Szmigiero , David Hildenbrand , Quentin Perret , Michael Roth , Wang , Liam Merwick , Isaku Yamahata , "Kirill A . Shutemov" Content-Type: text/plain; charset="us-ascii" X-Rspamd-Queue-Id: A1A2620016 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: 6bu1dyr9su87yz9hzep9rfxdtaph6gt6 X-HE-Tag: 1698846107-249228 X-HE-Meta: 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 E5TXr6st 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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 Wed, Nov 01, 2023, Xiaoyao Li wrote: > On 10/31/2023 10:16 PM, Sean Christopherson wrote: > > On Tue, Oct 31, 2023, Xiaoyao Li wrote: > > > On 10/28/2023 2:21 AM, Sean Christopherson wrote: > > > > Extended guest_memfd to allow backing guest memory with transparent > > > > hugepages. Require userspace to opt-in via a flag even though there's no > > > > known/anticipated use case for forcing small pages as THP is optional, > > > > i.e. to avoid ending up in a situation where userspace is unaware that > > > > KVM can't provide hugepages. > > > > > > Personally, it seems not so "transparent" if requiring userspace to opt-in. > > > > > > People need to 1) check if the kernel built with TRANSPARENT_HUGEPAGE > > > support, or check is the sysfs of transparent hugepage exists; 2)get the > > > maximum support hugepage size 3) ensure the size satisfies the alignment; > > > before opt-in it. > > > > > > Even simpler, userspace can blindly try to create guest memfd with > > > transparent hugapage flag. If getting error, fallback to create without the > > > transparent hugepage flag. > > > > > > However, it doesn't look transparent to me. > > > > The "transparent" part is referring to the underlying kernel mechanism, it's not > > saying anything about the API. The "transparent" part of THP is that the kernel > > doesn't guarantee hugepages, i.e. whether or not hugepages are actually used is > > (mostly) transparent to userspace. > > > > Paolo also isn't the biggest fan[*], but there are also downsides to always > > allowing hugepages, e.g. silent failure due to lack of THP or unaligned size, > > and there's precedent in the form of MADV_HUGEPAGE. > > > > [*] https://lore.kernel.org/all/84a908ae-04c7-51c7-c9a8-119e1933a189@redhat.com > > But it's different than MADV_HUGEPAGE, in a way. Per my understanding, the > failure of MADV_HUGEPAGE is not fatal, user space can ignore it and > continue. > > However, the failure of KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is fatal, which leads > to failure of guest memfd creation. Failing KVM_CREATE_GUEST_MEMFD isn't truly fatal, it just requires different action from userspace, i.e. instead of ignoring the error, userspace could redo KVM_CREATE_GUEST_MEMFD with KVM_GUEST_MEMFD_ALLOW_HUGEPAGE=0. We could make the behavior more like MADV_HUGEPAGE, e.g. theoretically we could extend fadvise() with FADV_HUGEPAGE, or add a guest_memfd knob/ioctl() to let userspace provide advice/hints after creating a guest_memfd. But I suspect that guest_memfd would be the only user of FADV_HUGEPAGE, and IMO a post-creation hint is actually less desirable. KVM_GUEST_MEMFD_ALLOW_HUGEPAGE will fail only if userspace didn't provide a compatible size or the kernel doesn't support THP. An incompatible size is likely a userspace bug, and for most setups that want to utilize guest_memfd, lack of THP support is likely a configuration bug. I.e. many/most uses *want* failures due to KVM_GUEST_MEMFD_ALLOW_HUGEPAGE to be fatal. > For current implementation, I think maybe KVM_GUEST_MEMFD_DESIRE_HUGEPAGE > fits better than KVM_GUEST_MEMFD_ALLOW_HUGEPAGE? or maybe *PREFER*? Why? Verbs like "prefer" and "desire" aren't a good fit IMO because they suggest the flag is a hint, and hints are usually best effort only, i.e. are ignored if there is a fundamental incompatibility. "Allow" isn't perfect, e.g. I would much prefer a straight KVM_GUEST_MEMFD_USE_HUGEPAGES or KVM_GUEST_MEMFD_HUGEPAGES flag, but I wanted the name to convey that KVM doesn't (yet) guarantee hugepages. I.e. KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is stronger than a hint, but weaker than a requirement. And if/when KVM supports a dedicated memory pool of some kind, then we can add KVM_GUEST_MEMFD_REQUIRE_HUGEPAGE.