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

Vishal Annapurve
2025-04-24T14:10:42Z

On Thu, Apr 24, 2025 at 1:15 AM Yan Zhao <yan.y.zhao@intel.com> wrote:
>
> 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.

Then during binding, guest memfd offset misalignment with hugepage
should be same as gfn misalignment. i.e.

(offset & ~huge_page_mask(h)) == ((slot->base_gfn << PAGE_SHIFT) &
~huge_page_mask(h));

For non guest_memfd backed scenarios, KVM allows slot gfn ranges that
are not hugepage aligned, so guest_memfd should also be able to
support non-hugepage aligned memslots.

> > >
> > >
> > >> 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;
> > >>>>> +}
> > >
> >