On Tue, 9 Mar 2021 19:45:03 -0400
Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> On Tue, Mar 09, 2021 at 12:56:39PM -0700, Alex Williamson wrote:
>
> > And I think this is what we end up with for the current code base:
>
> Yeah, that looks Ok
>
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index 65e7e6b44578..2f247ab18c66 100644
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -1568,19 +1568,24 @@ void vfio_pci_memory_unlock_and_restore(struct vfio_pci_device *vdev, u16 cmd)
> > }
> >
> > /* Caller holds vma_lock */
> > -static int __vfio_pci_add_vma(struct vfio_pci_device *vdev,
> > - struct vm_area_struct *vma)
> > +struct vfio_pci_mmap_vma *__vfio_pci_add_vma(struct vfio_pci_device *vdev,
> > + struct vm_area_struct *vma)
> > {
> > struct vfio_pci_mmap_vma *mmap_vma;
> >
> > + list_for_each_entry(mmap_vma, &vdev->vma_list, vma_next) {
> > + if (mmap_vma->vma == vma)
> > + return ERR_PTR(-EEXIST);
> > + }
> > +
> > mmap_vma = kmalloc(sizeof(*mmap_vma), GFP_KERNEL);
> > if (!mmap_vma)
> > - return -ENOMEM;
> > + return ERR_PTR(-ENOMEM);
> >
> > mmap_vma->vma = vma;
> > list_add(&mmap_vma->vma_next, &vdev->vma_list);
> >
> > - return 0;
> > + return mmap_vma;
> > }
> >
> > /*
> > @@ -1612,30 +1617,39 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
> > {
> > struct vm_area_struct *vma = vmf->vma;
> > struct vfio_pci_device *vdev = vma->vm_private_data;
> > - vm_fault_t ret = VM_FAULT_NOPAGE;
> > + struct vfio_pci_mmap_vma *mmap_vma;
> > + unsigned long vaddr, pfn;
> > + vm_fault_t ret;
> >
> > mutex_lock(&vdev->vma_lock);
> > down_read(&vdev->memory_lock);
> >
> > if (!__vfio_pci_memory_enabled(vdev)) {
> > ret = VM_FAULT_SIGBUS;
> > - mutex_unlock(&vdev->vma_lock);
> > goto up_out;
> > }
> >
> > - if (__vfio_pci_add_vma(vdev, vma)) {
> > - ret = VM_FAULT_OOM;
> > - mutex_unlock(&vdev->vma_lock);
> > + mmap_vma = __vfio_pci_add_vma(vdev, vma);
> > + if (IS_ERR(mmap_vma)) {
> > + /* A concurrent fault might have already inserted the page */
> > + ret = (PTR_ERR(mmap_vma) == -EEXIST) ? VM_FAULT_NOPAGE :
> > + VM_FAULT_OOM;
>
> I think -EEIXST should not be an error, lets just go down to the
> vmf_insert_pfn() and let the MM resolve the race naturally.
>
> I suspect returning VM_FAULT_NOPAGE will be averse to the userspace if
> it hits this race??
Given the serialization on vma_lock, if the vma_list entry exists then
the full vma should already be populated, so I don't see the NOPAGE
issue you're worried about. However, if we wanted to be more similar
to what we expect the new version to do, we could proceed through
re-inserting the pages on -EEXIST. Zeng Tao's re-ordering to add the
vma_list entry only after successfully inserting all the pfns might work
better for that.
> Also the _prot does look needed at least due to the SME, but possibly
> also to ensure NC gets set..
If we need more than pgprot_decrypted(vma->vm_page_prot), please let us
know, but that's all we were getting from io_remap_pfn_range() afaict.
Thanks,
Alex
