On Fri, 30 Aug 2019 00:58:22 +0800 Ben Luo <[email protected]> wrote:
> 在 2019/8/28 下午11:55, Alex Williamson 写道: > > On Wed, 28 Aug 2019 12:28:04 +0800 > > Ben Luo <[email protected]> wrote: > > > >> currently, if the page is not a tail of compound page, it will be > >> checked twice for the same thing. > >> > >> Signed-off-by: Ben Luo <[email protected]> > >> --- > >> drivers/vfio/vfio_iommu_type1.c | 3 +-- > >> 1 file changed, 1 insertion(+), 2 deletions(-) > >> > >> diff --git a/drivers/vfio/vfio_iommu_type1.c > >> b/drivers/vfio/vfio_iommu_type1.c > >> index 054391f..d0f7346 100644 > >> --- a/drivers/vfio/vfio_iommu_type1.c > >> +++ b/drivers/vfio/vfio_iommu_type1.c > >> @@ -291,11 +291,10 @@ static int vfio_lock_acct(struct vfio_dma *dma, long > >> npage, bool async) > >> static bool is_invalid_reserved_pfn(unsigned long pfn) > >> { > >> if (pfn_valid(pfn)) { > >> - bool reserved; > >> struct page *tail = pfn_to_page(pfn); > >> struct page *head = compound_head(tail); > >> - reserved = !!(PageReserved(head)); > >> if (head != tail) { > >> + bool reserved = PageReserved(head); > >> /* > >> * "head" is not a dangling pointer > >> * (compound_head takes care of that) > > Thinking more about this, the code here was originally just a copy of > > kvm_is_mmio_pfn() which was simplified in v3.12 with the commit below. > > Should we instead do the same thing here? Thanks, > > > > Alex > ok, and kvm_is_mmio_pfn() has also been updated since then, I will take > a look at that and compose a new patch I'm not sure if the further updates are quite as relevant for vfio, but appreciate your review of them. Thanks, Alex > > > > commit 11feeb498086a3a5907b8148bdf1786a9b18fc55 > > Author: Andrea Arcangeli <[email protected]> > > Date: Thu Jul 25 03:04:38 2013 +0200 > > > > kvm: optimize away THP checks in kvm_is_mmio_pfn() > > > > The checks on PG_reserved in the page structure on head and tail pages > > aren't necessary because split_huge_page wouldn't transfer the > > PG_reserved bit from head to tail anyway. > > > > This was a forward-thinking check done in the case PageReserved was > > set by a driver-owned page mapped in userland with something like > > remap_pfn_range in a VM_PFNMAP region, but using hugepmds (not > > possible right now). It was meant to be very safe, but it's overkill > > as it's unlikely split_huge_page could ever run without the driver > > noticing and tearing down the hugepage itself. > > > > And if a driver in the future will really want to map a reserved > > hugepage in userland using an huge pmd it should simply take care of > > marking all subpages reserved too to keep KVM safe. This of course > > would require such a hypothetical driver to tear down the huge pmd > > itself and splitting the hugepage itself, instead of relaying on > > split_huge_page, but that sounds very reasonable, especially > > considering split_huge_page wouldn't currently transfer the reserved > > bit anyway. > > > > Signed-off-by: Andrea Arcangeli <[email protected]> > > Signed-off-by: Gleb Natapov <[email protected]> > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > index d2836788561e..0fc25aed79a8 100644 > > --- a/virt/kvm/kvm_main.c > > +++ b/virt/kvm/kvm_main.c > > @@ -102,28 +102,8 @@ static bool largepages_enabled = true; > > > > bool kvm_is_mmio_pfn(pfn_t pfn) > > { > > - if (pfn_valid(pfn)) { > > - int reserved; > > - struct page *tail = pfn_to_page(pfn); > > - struct page *head = compound_trans_head(tail); > > - reserved = PageReserved(head); > > - if (head != tail) { > > - /* > > - * "head" is not a dangling pointer > > - * (compound_trans_head takes care of that) > > - * but the hugepage may have been splitted > > - * from under us (and we may not hold a > > - * reference count on the head page so it can > > - * be reused before we run PageReferenced), so > > - * we've to check PageTail before returning > > - * what we just read. > > - */ > > - smp_rmb(); > > - if (PageTail(tail)) > > - return reserved; > > - } > > - return PageReserved(tail); > > - } > > + if (pfn_valid(pfn)) > > + return PageReserved(pfn_to_page(pfn)); > > > > return true; > > }
