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

Yan Zhao
2025-04-24T08:15:58Z

On Thu, Apr 24, 2025 at 01:55:51PM +0800, Chenyi Qiang wrote:
>
>
> On 4/24/2025 12:25 PM, Yan Zhao wrote:
> > On Thu, Apr 24, 2025 at 09:09:22AM +0800, Yan Zhao wrote:
> >> 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);
>
> Should it be?
> lpage_info[2M][0].disallow_lpage = 1, which is for GFN [8K, 2M);
> lpage_info[2M][1].disallow_lpage = 0, which is for GFN [2M, 4M);
> lpage_info[2M][2].disallow_lpage = 0, which is for GFN [4M, 6M);
> lpage_info[2M][3].disallow_lpage = 0, which is for GFN [6M, 8M);
> lpage_info[2M][4].disallow_lpage = 1, which is for GFN [8M, 8M+8K);
Right. Good catch. Thanks!

Let me update the example as below:
slot->base_gfn is 2 (for GPA 8KB), npages 2000 (for a 8MB range)

lpage_info[2M][0].disallow_lpage = 1, which is for GPA [8KB, 2MB);
lpage_info[2M][1].disallow_lpage = 0, which is for GPA [2MB, 4MB);
lpage_info[2M][2].disallow_lpage = 0, which is for GPA [4MB, 6MB);
lpage_info[2M][3].disallow_lpage = 0, which is for GPA [6MB, 8MB);
lpage_info[2M][4].disallow_lpage = 1, which is for GPA [8MB, 8MB+8KB);

lpage_info indicates that a 2MB mapping is alllowed to cover GPA 4MB and GPA
4MB+16KB. However, their aligned_index values lead guest_memfd to allocate two
2MB folios, whose physical addresses may not be contiguous.

Additionally, if the guest accesses two GPAs, e.g., GPA 2MB+8KB and GPA 4MB,
KVM could create two 2MB mappings to cover GPA ranges [2MB, 4MB), [4MB, 6MB).
However, guest_memfd just allocates the same 2MB folio for both faults.

>
> >>
> >> ---------------------------------------------------------
> >> | | | | | | | | |
> >> 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.
> > Sorry, sent too fast this morning. The example is not right. The correct
> > one is:
> >
> > For GFN 4M and GFN 4M+16K, lpage_info indicates that 2M is allowed. So,
> > KVM will create a 2M mapping for them.
> >
> > However, in guest_memfd, GFN 4M and GFN 4M+16K do not correspond to the
> > same 2M folio and physical addresses may not be contiguous.
> >
> >
> >> 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;
> >>>>> +}
> >
>