Re: [PATCH] KVM: allow mapping of compound tail pages for IO or PFNMAP mapping

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 19, 2023, Yan Zhao wrote:
> Allow mapping of tail pages of compound pages for IO or PFNMAP mapping
> by trying and getting ref count of its head page.
> 
> For IO or PFNMAP mapping, sometimes it's backed by compound pages.
> KVM will just return error on mapping of tail pages of the compound pages,
> as ref count of the tail pages are always 0.
> 
> So, rather than check and add ref count of a tail page, check and add ref
> count of its folio (head page) to allow mapping of the compound tail pages.
> 
> This will not break the origial intention to disallow mapping of tail pages
> of non-compound higher order allocations as the folio of a non-compound
> tail page is the same as the page itself.
> 
> On the other side, put_page() has already converted page to folio before
> putting page ref.

Is there an actual use case for this?  It's not necessarily a strict requirement,
but it would be helpful to know if KVM supports this for a specific use case, or
just because it can.

Either way, this needs a selftest, KVM has had way too many bugs in this area.

> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> ---
>  virt/kvm/kvm_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 138292a86174..6f2b51ef20f7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2551,7 +2551,7 @@ static int kvm_try_get_pfn(kvm_pfn_t pfn)
>  	if (!page)
>  		return 1;
>  
> -	return get_page_unless_zero(page);
> +	return folio_try_get(page_folio(page));
>  }
>  
>  static int hva_to_pfn_remapped(struct vm_area_struct *vma,
> 
> base-commit: 24ff4c08e5bbdd7399d45f940f10fed030dfadda
> -- 
> 2.17.1
>