[PATCH v8 1/5] RDMA/umem: Support importing dma-buf as user memory region

[Jason Gunthorpe]

On Tue, Nov 10, 2020 at 03:14:45PM +0100, Daniel Vetter wrote:
> On Fri, Nov 06, 2020 at 12:39:53PM -0400, Jason Gunthorpe wrote:
> > On Fri, Nov 06, 2020 at 04:34:07PM +0000, Xiong, Jianxin wrote:
> > 
> > > > The user could specify a length that is beyond the dma buf, can
> > > > the dma buf length be checked during get?
> > > 
> > > In order to check the length, the buffer needs to be mapped. That can be done.
> > 
> > Do DMA bufs even have definitate immutable lengths? Going to be a
> > probelm if they can shrink
> 
> Yup. Unfortunately that's not document in the structures themselves,
> there's just some random comments sprinkled all over that dma-buf size is
> invariant, e.g. see the @mmap kerneldoc in dma_buf_ops:
> 
> https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_buf_ops#c.dma_buf_ops
> 
> "Because dma-buf buffers have invariant size over their lifetime, ..."
> 
> Jianxin, can you pls do a kerneldoc patch which updates the comment for
> dma_buf.size and dma_buf_export_info.size?

So we can check the size without doing an attachment?

That means the flow should be put back to how it was a series or two
ago where the SGL is only attached during page fault with the entire
programming sequence under the resv lock

Jason