Re: [PATCH v8 2/2] dma-buf: Split sgl by largest page-aligned chunk

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Sep 01, 2026 at 05:08:49PM +0000, David Hu wrote:
> From: David Hu <xuehaohu@google.com>
> 
> 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.

This patch is fine, but pedenatically a scatterlist's entry limit
should be bounded to dma_get_max_seg_size(), though I don't think it
helps this. Operating scatterlists at the size limits has proven
problematic in a number of places already...

> In addition, a non-page-aligned sgl length will trigger an edge case
> in `ib_umem_find_best_pgsz()`. In case of a discontinuity in later
> buffers, we will have a `va` with lowest bit set to 1. That will lead
> to `ib_umem_find_best_pgsz()` always return 0, and break the promise
> to find best page size for the mapping on the NIC side.

That's an IB side bug, the newer logic that joins adjacent SGLs should
have avoided it?

Jason