Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for dedicated guest memory

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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.


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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