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

Yan Zhao
2025-04-24T04:27:37Z

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);
>
> ---------------------------------------------------------
> | | | | | | | | |
> 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;
> > >> +}