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!
