On Wed, Nov 01, 2023, Paolo Bonzini wrote: > On Wed, Nov 1, 2023 at 2:41 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > 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: > > > 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. > > I think that the current patch is fine, but I will adjust it to always > allow the flag, and to make the size check even if !CONFIG_TRANSPARENT_HUGEPAGE. > If hugepages are not guaranteed, and (theoretically) you could have no > hugepage at all in the result, it's okay to get this result even if THP is not > available in the kernel. Can you post a fixup patch? It's not clear to me exactly what behavior you intend to end up with.
