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

[Yan Zhao]

On Mon, Feb 12, 2024 at 07:17:21PM -0800, Sean Christopherson wrote:
> 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).
Sure.

> 
> > 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@xxxxxxxxx>
> > ---
> >  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).
Yes, you are right!
Will do the retry. Thanks!