[Feature Request] Support hugetlbfs memory files for snapshot creation and file-backed restore

Feature Request

Allow Firecracker to create snapshot memory files on a 2 MiB hugetlbfs mount and restore those files directly using the File memory backend.

Currently, Firecracker supports explicit 2 MiB hugetlbfs pages for newly created microVMs. It can also restore a snapshot into hugetlbfs-backed anonymous memory through UFFD.

However:

• The File snapshot backend explicitly rejects huge_pages: "2M".
• The snapshot writer cannot populate a memory file located on hugetlbfs using its current ordinary file-I/O path.
• As a result, explicit hugetlbfs snapshot restore always requires a separate UFFD handler.

UFFD is useful for demand paging, but it requires a separate userspace process and fault-handling protocol. For fully resident snapshots reused by one or more microVMs, a hugetlbfs memory file could instead be mapped directly.

Describe the desired solution

Snapshot creation

When mem_file_path for PUT /snapshot/create refers to a file on a compatible hugetlbfs mount, Firecracker would:

1. Size the file and map it writable with MAP_SHARED.
2. Write the full or differential memory snapshot through that mapping.

Snapshot restore

When PUT /snapshot/load uses a File backend with huge_pages: "2M", Firecracker would map a compatible hugetlbfs memory file read-write with MAP_PRIVATE.

This allows guest writes through copy-on-write without modifying the snapshot file. Existing behavior for regular files and UFFD would remain unchanged.

Describe possible alternatives

The current workaround is to restore through UFFD, which requires running and managing a separate page-fault handler.

Transparent huge pages can be used with file-backed restore, but they are opportunistic and do not provide deterministic backing from the explicit hugetlbfs pool.

Additional context

Checks

• Have you searched the Firecracker Issues database for similar requests?
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