Re: [RFC PATCH 39/39] KVM: guest_memfd: Dynamically split/reconstruct HugeTLB page

Yan Zhao
2025-04-24T01:11:43Z

On Wed, Apr 23, 2025 at 03:02:02PM -0700, Ackerley Tng wrote:
> Yan Zhao <yan.y.zhao@intel.com> writes:
>
> > On Tue, Sep 10, 2024 at 11:44:10PM +0000, Ackerley Tng wrote:
> >> +/*
> >> + * Allocates and then caches a folio in the filemap. Returns a folio with
> >> + * refcount of 2: 1 after allocation, and 1 taken by the filemap.
> >> + */
> >> +static struct folio *kvm_gmem_hugetlb_alloc_and_cache_folio(struct inode *inode,
> >> + pgoff_t index)
> >> +{
> >> + struct kvm_gmem_hugetlb *hgmem;
> >> + pgoff_t aligned_index;
> >> + struct folio *folio;
> >> + int nr_pages;
> >> + int ret;
> >> +
> >> + hgmem = kvm_gmem_hgmem(inode);
> >> + folio = kvm_gmem_hugetlb_alloc_folio(hgmem->h, hgmem->spool);
> >> + if (IS_ERR(folio))
> >> + return folio;
> >> +
> >> + nr_pages = 1UL << huge_page_order(hgmem->h);
> >> + aligned_index = round_down(index, nr_pages);
> > Maybe a gap here.
> >
> > When a guest_memfd is bound to a slot where slot->base_gfn is not aligned to
> > 2M/1G and slot->gmem.pgoff is 0, even if an index is 2M/1G aligned, the
> > corresponding GFN is not 2M/1G aligned.
>
> Thanks for looking into this.
>
> In 1G page support for guest_memfd, the offset and size are always
> hugepage aligned to the hugepage size requested at guest_memfd creation
> time, and it is true that when binding to a memslot, slot->base_gfn and
> slot->npages may not be hugepage aligned.
>
> >
> > However, TDX requires that private huge pages be 2M aligned in GFN.
> >
>
> IIUC other factors also contribute to determining the mapping level in
> the guest page tables, like lpage_info and .private_max_mapping_level()
> in kvm_x86_ops.
>
> If slot->base_gfn and slot->npages are not hugepage aligned, lpage_info
> will track that and not allow faulting into guest page tables at higher
> granularity.

lpage_info only checks the alignments of slot->base_gfn and
slot->base_gfn + npages. e.g.,

if slot->base_gfn is 8K, npages is 8M, then for this slot,
lpage_info[2M][0].disallow_lpage = 1, which is for GFN [4K, 2M+8K);
lpage_info[2M][1].disallow_lpage = 0, which is for GFN [2M+8K, 4M+8K);
lpage_info[2M][2].disallow_lpage = 0, which is for GFN [4M+8K, 6M+8K);
lpage_info[2M][3].disallow_lpage = 1, which is for GFN [6M+8K, 8M+8K);

---------------------------------------------------------
| | | | | | | | |
8K 2M 2M+8K 4M 4M+8K 6M 6M+8K 8M 8M+8K

For GFN 6M and GFN 6M+4K, as they both belong to lpage_info[2M][2], huge
page is allowed. Also, they have the same aligned_index 2 in guest_memfd.
So, guest_memfd allocates the same huge folio of 2M order for them.

However, for TDX, GFN 6M and GFN 6M+4K should not belong to the same folio.
It's also weird for a 2M mapping in KVM to stride across 2 huge folios.

> Hence I think it is okay to leave it to KVM to fault pages into the
> guest correctly. For guest_memfd will just maintain the invariant that
> offset and size are hugepage aligned, but not require that
> slot->base_gfn and slot->npages are hugepage aligned. This behavior will
> be consistent with other backing memory for guests like regular shmem or
> HugeTLB.
>
> >> + ret = kvm_gmem_hugetlb_filemap_add_folio(inode->i_mapping, folio,
> >> + aligned_index,
> >> + htlb_alloc_mask(hgmem->h));
> >> + WARN_ON(ret);
> >> +
> >> spin_lock(&inode->i_lock);
> >> inode->i_blocks += blocks_per_huge_page(hgmem->h);
> >> spin_unlock(&inode->i_lock);
> >>
> >> - return page_folio(requested_page);
> >> + return folio;
> >> +}