[Linaro-mm-sig] Re: [PATCH v3 2/9] vfio/pci: Add a helper to look up PFNs for DMABUFs

From: Matt Evans matt@ozlabs.org Sent: Wednesday, June 10, 2026 11:43 PM

+int vfio_pci_dma_buf_find_pfn(struct vfio_pci_dma_buf *priv,

• 	      struct vm_area_struct *vma,
  

• 	      unsigned long address,
  

• 	      unsigned int order,
  

• 	      unsigned long *out_pfn)
  

+{

• /*

• * Given a VMA (start, end, pgoffs) and a fault address,
  

• * search the corresponding DMABUF's phys_vec[] to find the
  

• * range representing the address's offset into the VMA, and
  

• * its PFN.
  

• *
  

• * The phys_vec[] ranges represent contiguous spans of VAs
  

• * upwards from the buffer offset 0; the actual PFNs might be
  

• * in any order, overlap/alias, etc.  Calculate an offset of
  

• * the desired page given VMA start/pgoff and address, then
  

• * search upwards from 0 to find which span contains it.
  

• *
  

• * On success, a valid PFN for a page sized by 'order' is
  

• * returned into out_pfn.
  

• *
  

• * Failure occurs if:
  

• * - The page would cross the edge of the VMA
  

• * - The page isn't entirely contained within a range
  

• * - We find a range, but the final PFN isn't aligned to the
  

• *   requested order.
  

• *
  

• * (Upon failure, the caller is expected to try again with a
  

• * smaller order; the tests above will always succeed for
  

• * order=0 as the limit case.)
  

• *
  

• * It's suboptimal if DMABUFs are created with neigbouring
  

s/neigbouring/neighboring/

• * ranges that are physically contiguous, since hugepages
  

• * can't straddle range boundaries.  (The construction of the
  

• * ranges vector should merge such ranges.)
  

though the field is called 'phys_vec', removing 'vector' in description is clearer here.

• *
  

• * Finally, vma_pgoff_adjust is used for a DMABUF representing
  

• * a VFIO BAR mmap, which is created from the start of the
  

• * offset region.
  

Elaborate it a little bit that the vm_pgoff is already counted in paddr of phys_vec so it should be skipped when finding the pfn.

• */
  

• const unsigned long pagesize = PAGE_SIZE << order;
• unsigned long vma_off = ((vma->vm_pgoff - priv->vma_pgoff_adjust)

<<

• 		 PAGE_SHIFT) & VFIO_PCI_OFFSET_MASK;
  

• unsigned long rounded_page_addr = ALIGN_DOWN(address,

pagesize);

• unsigned long rounded_page_end = rounded_page_addr + pagesize;
• unsigned long page_buf_offset;
• unsigned long page_buf_offset_end;

what about "fault_offset[_end]"? page_buf is a bit confusing.

• unsigned long range_buf_offset = 0;

could this be called 'range_start' then the 'range_start' in latter loop is renamed to 'phys_start'?

Not strong... just feel such naming helps me understand the logic easier

• unsigned int i;
• if (rounded_page_addr < vma->vm_start || rounded_page_end >

vma->vm_end) {

• if (order > 0)
  

• 	return -EAGAIN;
  

• /* A fault address outside of the VMA is absurd. */
  

• WARN(1, "Fault addr 0x%lx outside VMA 0x%lx-0x%lx\n",
  

•      address, vma->vm_start, vma->vm_end);
  

• return -EFAULT;
  

• }
• /*

• * page_buff_offset[_end] is the span of DMABUF offsets
  

• * corresponding to the faulting page:
  

• */
  

if the naming is kept then s/page_buff_offset/page_buf_offset/

otherwise,

Reviewed-by: Kevin Tian kevin.tian@intel.com