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

[Jason Gunthorpe]

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 <mrgolin@xxxxxxxxxx>
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