On Tue, 28 Apr 2020 14:49:57 -0300
Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> On Tue, Apr 28, 2020 at 10:54:55AM -0600, Alex Williamson wrote:
> > static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
> > {
> > struct vfio_pci_device *vdev = device_data;
> > @@ -1253,8 +1323,14 @@ static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
> > vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> > vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
> >
> > + vma->vm_ops = &vfio_pci_mmap_ops;
> > +
> > +#if 1
> > + return 0;
> > +#else
> > return remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
> > - req_len, vma->vm_page_prot);
> > + vma->vm_end - vma->vm_start, vma->vm_page_prot);
>
> The remap_pfn_range here is what tells get_user_pages this is a
> non-struct page mapping:
>
> vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
>
> Which has to be set when the VMA is created, they shouldn't be
> modified during fault.
Aha, thanks Jason! So fundamentally, pin_user_pages_remote() should
never have been faulting in this vma since the pages are non-struct
page backed. Maybe I was just getting lucky before this commit. For a
VM_PFNMAP, vaddr_get_pfn() only needs pin_user_pages_remote() to return
error and the vma information that we setup in vfio_pci_mmap(). We
only need the fault handler to trigger for user access, which is what I
see with this change. That should work for me.
> Also the vma code above looked a little strange to me, if you do send
> something like this cc me and I can look at it. I did some work like
> this for rdma a while ago..
Cool, I'll do that. I'd like to be able to zap the vmas from user
access at a later point and I have doubts that I'm holding the
refs/locks that I need to for that. Thanks,
Alex
