Re: [LSF/MM/BPF TOPIC] dmabuf backed read/write

Jason Gunthorpe
2026-02-05T23:57:05Z

On Thu, Feb 05, 2026 at 07:06:03PM +0000, Pavel Begunkov wrote:
> On 2/5/26 17:41, Jason Gunthorpe wrote:
> > On Tue, Feb 03, 2026 at 02:29:55PM +0000, Pavel Begunkov wrote:
> >
> > > The proposal consists of two parts. The first is a small in-kernel
> > > framework that allows a dma-buf to be registered against a given file
> > > and returns an object representing a DMA mapping.
> >
> > What is this about and why would you need something like this?
> >
> > The rest makes more sense - pass a DMABUF (or even memfd) to iouring
> > and pre-setup the DMA mapping to get dma_addr_t, then directly use
> > dma_addr_t through the entire block stack right into the eventual
> > driver.
>
> That's more or less what I tried to do in v1, but 1) people didn't like
> the idea of passing raw dma addresses directly, and having it wrapped
> into a black box gives more flexibility like potentially supporting
> multi-device filesystems.

Ok.. but what does that have to do with a user space visible file?

> 2) dma-buf folks want dynamic attachments,
> and it makes it quite a bit more complicated when you might be asked to
> shoot down DMA mappings at any moment, so I'm isolating all that
> into something that can be reused.

IMHO there is probably nothing really resuable here. The logic to
fence any usage is entirely unique to whoever is using it, and the
locking tends to be really hard.

You should review the email threads linked to this patch and all it's
prior versions as the expected importer behavior for pinned dmabufs is
not well understood.

https://lore.kernel.org/all/20260131-dmabuf-revoke-v7-0-463d956bd527@nvidia.com/

> > > Tushar was helping and mention he got good numbers for P2P transfers
> > > compared to bouncing it via RAM.
> >
> > We can already avoid the bouncing, it seems the main improvements here
> > are avoiding the DMA map per-io and allowing the use of P2P without
> > also creating struct page. Meanginful wins for sure.
>
> Yes, and it should probably be nicer for frameworks that already
> expose dma-bufs.

I'm not sure what this means?

Jason