Re: [Intel-gfx] [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64 MMU

Paolo Bonzini
2021-06-24T12:00:37Z

On 24/06/21 13:42, Nicholas Piggin wrote:
> +static int kvm_try_get_pfn(kvm_pfn_t pfn)
> +{
> + if (kvm_is_reserved_pfn(pfn))
> + return 1;

So !pfn_valid would always return true. Yeah, this should work and is
certainly appealing!

Paolo

> + return get_page_unless_zero(pfn_to_page(pfn));
> +}
> +
> static int hva_to_pfn_remapped(struct vm_area_struct *vma,
> unsigned long addr, bool *async,
> bool write_fault, bool *writable,
> @@ -2104,13 +2111,21 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
> * Whoever called remap_pfn_range is also going to call e.g.
> * unmap_mapping_range before the underlying pages are freed,
> * causing a call to our MMU notifier.
> + *
> + * Certain IO or PFNMAP mappings can be backed with valid
> + * struct pages, but be allocated without refcounting e.g.,
> + * tail pages of non-compound higher order allocations, which
> + * would then underflow the refcount when the caller does the
> + * required put_page. Don't allow those pages here.
> */
> - kvm_get_pfn(pfn);
> + if (!kvm_try_get_pfn(pfn))
> + r = -EFAULT;
>
> out:
> pte_unmap_unlock(ptep, ptl);
> *p_pfn = pfn;
> - return 0;
> +
> + return r;
> }
>
> /*