Re: [PATCH v2] dma-buf: Split sgl into page-aligned 2G chunks

[Leon Romanovsky]

On Tue, Jun 23, 2026 at 11:53:50PM +0100, David Laight wrote:
> On Tue, 23 Jun 2026 20:55:32 +0000
> Pranjal Shrivastava <praan@xxxxxxxxxx> wrote:
> 
> > On Tue, Jun 23, 2026 at 09:44:46AM +0100, David Laight wrote:
> > 
> > Hi David,
> > 
> > > On Tue, 23 Jun 2026 01:54:59 +0000
> > > David Hu <xuehaohu@xxxxxxxxxx> wrote:
> > >   
> > > > Currently, `fill_sg_entry()` splits the scatterlist using `UINT_MAX`.
> > > > This creates a non-page-aligned DMA length (`0xFFFFFFFF`) for the
> > > > first entry, resulting in non-page-aligned DMA addresses for all
> > > > subsequent entries.  
> > > 
> > > There is a separate issue of whether this code is even needed at all.
> > > Where can transfers over 2G (never mind 4G) actually come from.
> > > 
> > > The read, write and similar system calls limit transfers to INT_MAX
> > > (even on 64bit) and a lot of driver code will need fixing it longer
> > > lengths are allowed though.
> > > io_uring better enforce the same limits.
> > > So the transfers can come directly from userspace.
> > > 
> > > Not only that but you also need a single physically contiguous buffer.
> > > Good luck allocating that!
> > > 
> > > Now maybe there are some peer-to-peer places where the large buffer
> > > is device memory, but they will be unusual and probably need
> > > special treatment anyway.
> > >   
> > 
> > I agree that traditional VFS read/write face the MAX_RW_COUNT limit 
> > (~2GB), and io_uring has its limits, but I'm a little confused by the
> > push to enforce these limits here in the SGL code?
> > 
> > File I/O seems to be only one side of the picture. In my view, this fix
> > is necessary and certainly has a use-case:
> > 
> > For example, the RDMA subsystem has the capability to import dmabufs [1],
> > which gives rise to use cases for dmabuf beyond standard file ops 
> > (via VFS/io_uring). 
> > 
> > In these scenarios, GPU HBM can be exported as dmabufs. With recent GPUs,
> > HBM capacity can be in the order of hundreds of GBs [2]. RDMA can employ
> > infrastructure like the vfio-dmabuf-exporter [3] or similar dmabuf 
> > exporters to frequently move huge blocks of data via P2PDMA.
> 
> Ok, that explains where big buffers can come from.
> I just wasn't sure.
> 
> > If we restrict incoming dmabuf transfers to fit within VFS-centric 
> > limits (2GB), we impose unnecessary overhead on the RDMA stack, forcing
> > it to manage a significantly higher number of memory registrations. By 
> > cleanly splitting these massive contiguous device buffers into 
> > page-aligned SGL entries, we directly improve the efficiency of P2P 
> > transfers and memory registration.
> 
> But a divide by '4G - PAGE_SIZE' is also non-trivial and (I think affects
> a lot of io) when the quotient is always 1.
> Splitting into 2G chunks is a lot cheaper.
> 
> > Since this change doesn't seem to have a negative impact on standard file
> > I/O or break existing VFS constraints, I'm curious why we shouldn't 
> > support splitting these >4GB P2P transfers? Am I missing something?
> 
> I was only wondering whether it was needed...
> It does bring up the question of why the >4GB transfers even need splitting.
> But that is another question.

Just a side note:

In our vision, we aim to transition DMABUF to use physical  
addresses directly https://lore.kernel.org/all/0-v1-b5cab63049c0+191af-dmabuf_map_type_jgg@xxxxxxxxxx/  
and eliminate the scatter‑gather layer from the DMABUF path.

Thanks

> 
> If you want to split large transfers into 4G-PAGE_SIZE blocks
> it is probably worth having a quick test that returns 1 for 'small' buffers.
> 
> 	David
> 
> > 
> > Thanks,
> > Praan
> > 
> > [1] https://elixir.bootlin.com/linux/v7.1.1/source/drivers/infiniband/core/umem_dmabuf.c#L174 
> > [2] https://nvdam.widen.net/s/fdvdqvfvj2/hopper-h200-nvl-product-brief (Table 2-2)
> > [3] https://elixir.bootlin.com/linux/v7.1.1/source/drivers/vfio/pci/vfio_pci_dmabuf.c#L297
> > 
>