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. 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 >
