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

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Message-ID: <aAmPQssuN9Zba//b@yzhao56-desk.sh.intel.com>
Date: Thu, 24 Apr 2025 09:09:22 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH 39/39] KVM: guest_memfd: Dynamically
 split/reconstruct HugeTLB page

On Wed, Apr 23, 2025 at 03:02:02PM -0700, Ackerley Tng wrote:
> Yan Zhao <yan.y.zhao@...el.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;
> >> +}