| Date | Wed, 1 Nov 2023 15:25:23 +0800 | Subject | Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for dedicated guest memory | From | Xiaoyao Li <> |
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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.
For current implementation, I think maybe KVM_GUEST_MEMFD_DESIRE_HUGEPAGE fits better than KVM_GUEST_MEMFD_ALLOW_HUGEPAGE? or maybe *PREFER*?
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