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@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 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 >
