On Wed, May 08, 2024 at 06:02:14PM +0100, Pavel Begunkov wrote:

Well, the example fell flat, but you don't use dmabuf when there are
no upsides from using it. For instance, when you already have pinned
pages, you're going to use pages, and there are no other refcounting
concerns.

Sure.

Unless there is an advantage of dmabufs over FOLL_LONGTERM
that I don't know about when used with normal user pages.

The advantages of using a dma-buf over FOLL_LONGTERM are:

 a) you pre-dma map, which is a significant performance advantage for
    IOMMU-based setups
 b) you support any dma-buf exported and not just user memory.  This
    is primarily important for PCIe P2P, but there might be other
    useful exporters as well

quoted

wish io_uring would have just implemented them from the start instead of
the current fixed buffers that are not quite as useful by not
pre-mapping DMA and not supporting P2P.

fdget(dmabuf) would be horrible, I assume that's not the suggestion.

I'm not even sure what you mean with that.