Re: [PATCH RFC v1 0/5] KVM: gmem: 2MB THP support and preparedness tracking changes

[Vishal Annapurve <vannapurve@google.com>]

On Wed, Dec 11, 2024 at 10:37 PM Michael Roth <michael.roth@amd.com> wrote:
>
> This patchset is also available at:
>
>   https://github.com/amdese/linux/commits/snp-prepare-thp-rfc1
>
> and is based on top of Paolo's kvm-coco-queue-2024-11 tag which includes
> a snapshot of his patches[1] to provide tracking of whether or not
> sub-pages of a huge folio need to have kvm_arch_gmem_prepare() hooks issued
> before guest access:
>
>   d55475f23cea KVM: gmem: track preparedness a page at a time
>   64b46ca6cd6d KVM: gmem: limit hole-punching to ranges within the file
>   17df70a5ea65 KVM: gmem: add a complete set of functions to query page preparedness
>   e3449f6841ef KVM: gmem: allocate private data for the gmem inode
>
>   [1] https://lore.kernel.org/lkml/20241108155056.332412-1-pbonzini@redhat.com/
>
> This series addresses some of the pending review comments for those patches
> (feel free to squash/rework as-needed), and implements a first real user in
> the form of a reworked version of Sean's original 2MB THP support for gmem.
>

Looking at the work targeted by Fuad to add in-place memory conversion
support via [1] and Ackerley in future to address hugetlb page
support, can the state tracking for preparedness be simplified as?
i) prepare guest memfd ranges when "first time an offset with
mappability = GUEST is allocated or first time an allocated offset has
mappability = GUEST". Some scenarios that would lead to guest memfd
range preparation:
     - Create file with default mappability to host, fallocate, convert
     - Create file with default mappability to Guest, guest faults on
private memory
ii) Unprepare guest memfd ranges when "first time an offset with
mappability = GUEST is deallocated or first time an allocated offset
has lost mappability = GUEST attribute", some scenarios that would
lead to guest memfd range unprepare:
     -  Truncation
     -  Conversion
iii) To handle scenarios with hugepages, page splitting/merging in
guest memfd can also signal change in page granularities.

[1] https://lore.kernel.org/kvm/20250117163001.2326672-1-tabba@google.com/