On Tue, Jun 23, 2026 at 11:53:50PM +0100, David Laight wrote: > On Tue, 23 Jun 2026 20:55:32 +0000 > Pranjal Shrivastava <[email protected]> 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 <[email protected]> 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/[email protected]/ 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 > > >
