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-c9a8-119e1933a...@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.
