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. 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. 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? 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
