On Tue, Jun 23, 2026 at 11:53:50PM +0100, David Laight wrote: > > 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.
Doesn't matter this isn't fast path stuff. It is better to use fewer SGL entries, IHMO. > > 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. SGL can only store an unsigned int size, so any large physical range has to be split down. rdma now a days has code to process the sgl and restore back the > 4G sizes since mode RDMA HW can accept that. commit 486055f5e09df959ad4e3aa4ee75b5c91ddeec2e Author: Michael Margolin <[email protected]> Date: Mon Feb 17 14:16:23 2025 +0000 RDMA/core: Fix best page size finding when it can cross SG entries So whatever this produces needs to be compatible with that to undo it. Jason
