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@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).
