On Thu, Nov 20, 2025 at 08:47:20PM -0400, Jason Gunthorpe wrote:

On Thu, Nov 20, 2025 at 08:04:37AM +0000, Tian, Kevin wrote:

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From: Jason Gunthorpe jgg@nvidia.com Sent: Saturday, November 8, 2025 12:50 AM

+static int pfn_reader_fill_dmabuf(struct pfn_reader_dmabuf *dmabuf,

• 		  struct pfn_batch *batch,
  

• 		  unsigned long start_index,
  

• 		  unsigned long last_index)
  

+{

• unsigned long start = dmabuf->start_offset + start_index * PAGE_SIZE;
• /*

• * This works in PAGE_SIZE indexes, if the dmabuf is sliced and
  

• * starts/ends at a sub page offset then the batch to domain code will
  

• * adjust it.
  

• */
  

dmabuf->start_offset comes from pages->dmabuf.start, which is initialized as:

pages->dmabuf.start = start - start_byte;

so it's always page-aligned. Where is the sub-page offset coming from?

I need to go over this again to check it, this sub-page stuff is a bit convoluted. start_offset should include the sub page offset here..

So in the pfn reader section everything is page aligned, that means the start location is also page aligned. The comment is just a bit confusing, maybe this is better:

/* * start/last_index and start_offset are all PAGE_SIZE aligned, the * batch is always filled using page size aligned PFNs just like the * other types. If the dmabuf has been sliced with a sub page offset * then the common batch to domain code will adjust it before mapping to * the domain. */

The adjustment is this code in batch_to_domain():

/* The first index might be a partial page */ if (start_index == iopt_area_index(area)) page_offset = area->page_offset;

Thanks, Jason