Re: [PATCH 2/3] KVM: gmem: add a complete set of functions to query page preparedness

[Sean Christopherson <seanjc@google.com>]

On Fri, Nov 08, 2024, Paolo Bonzini wrote:
> In preparation for moving preparedness out of the folio flags, pass
> the struct file* or struct inode* down to kvm_gmem_mark_prepared,
> as well as the offset within the gmem file.  Introduce new functions
> to unprepare page on punch-hole, and to query the state.

...

> +static bool kvm_gmem_is_prepared(struct file *file, pgoff_t index, struct folio *folio)
> +{
> +	return folio_test_uptodate(folio);
> +}
> +
>  /*
>   * Process @folio, which contains @gfn, so that the guest can use it.
>   * The folio must be locked and the gfn must be contained in @slot.
>   * On successful return the guest sees a zero page so as to avoid
>   * leaking host data and the up-to-date flag is set.
>   */
> -static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
> +static int kvm_gmem_prepare_folio(struct kvm *kvm, struct file *file,
> +				  struct kvm_memory_slot *slot,
>  				  gfn_t gfn, struct folio *folio)
>  {
>  	unsigned long nr_pages, i;
> @@ -147,7 +157,7 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
>  	index = ALIGN_DOWN(index, 1 << folio_order(folio));
>  	r = __kvm_gmem_prepare_folio(kvm, slot, index, folio);
>  	if (!r)
> -		kvm_gmem_mark_prepared(folio);
> +		kvm_gmem_mark_prepared(file, index, folio);
>  
>  	return r;
>  }
> @@ -231,6 +241,7 @@ static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
>  		kvm_gmem_invalidate_begin(gmem, start, end);
>  
>  	truncate_inode_pages_range(inode->i_mapping, offset, offset + len - 1);
> +	kvm_gmem_mark_range_unprepared(inode, start, end - start);
>  
>  	list_for_each_entry(gmem, gmem_list, entry)
>  		kvm_gmem_invalidate_end(gmem, start, end);
> @@ -682,7 +693,7 @@ __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
>  	if (max_order)
>  		*max_order = 0;
>  
> -	*is_prepared = folio_test_uptodate(folio);
> +	*is_prepared = kvm_gmem_is_prepared(file, index, folio);
>  	return folio;
>  }
>  
> @@ -704,7 +715,7 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
>  	}
>  
>  	if (!is_prepared)
> -		r = kvm_gmem_prepare_folio(kvm, slot, gfn, folio);
> +		r = kvm_gmem_prepare_folio(kvm, file, slot, gfn, folio);

This is broken when the next patch comes along.  If KVM encounters a partially
prepared folio, i.e. a folio with some prepared pages and some unprepared pages,
then KVM needs to zero only the unprepared pages.  But kvm_gmem_prepare_folio()
zeroes everything.

static int kvm_gmem_prepare_folio(struct kvm *kvm, struct file *file,
				  struct kvm_memory_slot *slot,
				  gfn_t gfn, struct folio *folio)
{
	unsigned long nr_pages, i;
	pgoff_t index;
	int r;

	nr_pages = folio_nr_pages(folio);
	for (i = 0; i < nr_pages; i++)
		clear_highpage(folio_page(folio, i));