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

[Yan Zhao]

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?
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)) {