Re: [RFC PATCH v2 1/3] KVM: allow mapping of compound tail pages for IO or PFNMAP mapping

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 03, 2024, 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.

Can you add a blurb to call out that this is effectively what gup() does in
try_get_folio()?  That knowledge give me a _lot_ more confidence that this is
correct (I didn't think too deeply about what this patch was doing when I looked
at v1).

> 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.
> 
> 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 acd67fb40183..f53b58446ac7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2892,7 +2892,7 @@ 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));

This seems like it needs retry logic, a la try_get_folio(), to guard against a
race with the folio being split.  From page_folio():
  
 If the caller* does not hold a reference, this call may race with a folio split,
 so it should re-check the folio still contains this page after gaining a
 reference on the folio.

I assume that splitting one of these folios is extremely unlikely, but I don't
see any harm in being paranoid (unless this really truly cannot race).