Re: [PATCH] vfio/pci: take mmap write lock for io_remap_pfn_range

[Cédric Le Goater]

On 5/11/23 08:56, Yan Zhao wrote:
> On Wed, May 10, 2023 at 05:41:06PM -0300, Jason Gunthorpe wrote:
> > On Mon, May 08, 2023 at 02:57:15PM -0600, Alex Williamson wrote:
> >
> > > We already try to set the flags in advance, but there are some
> > > architectural flags like VM_PAT that make that tricky.  Cedric has been
> > > looking at inserting individual pages with vmf_insert_pfn(), but that
> > > incurs a lot more faults and therefore latency vs remapping the entire
> > > vma on fault.  I'm not convinced that we shouldn't just attempt to
> > > remove the fault handler entirely, but I haven't tried it yet to know
> > > what gotchas are down that path.  Thanks,
> >
> > I thought we did it like this because there were races otherwise with
> > PTE insertion and zapping? I don't remember well anymore.
> >
> > I vaugely remember the address_space conversion might help remove the
> > fault handler?
> What about calling vmf_insert_pfn() in bulk as below?

This works too, it is slightly slower than the io_remap_pfn_range() call
but doesn't have the lockdep issues.

Thanks,

C.

> And what is address_space conversion?
>
>
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index a5ab416cf476..1476e537f593 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1686,6 +1686,7 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
>          struct vfio_pci_core_device *vdev = vma->vm_private_data;
>          struct vfio_pci_mmap_vma *mmap_vma;
>          vm_fault_t ret = VM_FAULT_NOPAGE;
> +       unsigned long base_pfn, offset, i;
>
>          mutex_lock(&vdev->vma_lock);
>          down_read(&vdev->memory_lock);
> @@ -1710,12 +1711,15 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
>                          goto up_out;
>          }
>
> -       if (io_remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
> -                              vma->vm_end - vma->vm_start,
> -                              vma->vm_page_prot)) {
> -               ret = VM_FAULT_SIGBUS;
> -               zap_vma_ptes(vma, vma->vm_start, vma->vm_end - vma->vm_start);
> -               goto up_out;
> +       base_pfn = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
> +       base_pfn += vma->vm_pgoff;
> +       for (i = vma->vm_start; i < vma->vm_end; i += PAGE_SIZE) {
> +               offset = (i - vma->vm_start) >> PAGE_SHIFT;
> +               ret = vmf_insert_pfn(vma, i, base_pfn + offset);
> +               if (ret != VM_FAULT_NOPAGE) {
> +                       zap_vma_ptes(vma, vma->vm_start, vma->vm_end - vma->vm_start);
> +                       goto up_out;
> +               }
>          }
>
>          if (__vfio_pci_add_vma(vdev, vma)) {