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

[Xiaoyao Li]

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@xxxxxxxxxx

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*?