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

Ackerley Tng
2025-04-24T18:15:14Z

Vishal Annapurve <vannapurve@google.com> writes:

> 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.
>

I drew up a picture [1] which hopefully clarifies this.

Thanks for pointing this out, I understand better now and we will add an
extra constraint during memslot binding of guest_memfd to check that gfn
offsets within a hugepage must be guest_memfd offsets.

Adding checks at binding time will allow hugepage-unaligned offsets (to
be at parity with non-guest_memfd backing memory) but still fix this
issue.

lpage_info will make sure that ranges near the bounds will be
fragmented, but the hugepages in the middle will still be mappable as
hugepages.

[1] https://lpc.events/event/18/contributions/1764/attachments/1409/3706/binding-must-have-same-alignment.svg

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