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

Chenyi Qiang
2025-04-24T05:56:54Z

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);

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