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

[David Hu]

On Wed, Sep 2, 2026 at 8:08 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Tue, Sep 01, 2026 at 05:08:49PM +0000, David Hu wrote:
> > From: David Hu <xuehaohu@xxxxxxxxxx>
> >
> > 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?

Hi Jason,

Thank you for the review. I think you are right on both counts.
Regarding the IB side, the new SGL joining logic in
`ib_umem_find_best_pgsz()` indeed avoids the issue. `mask |= va` is
skipped for artifically split, contiguous SGLs. Since Christian has
already pulled v8 into drm-misc-next, I won't spin a v9 to avoid
creating unnecessary noise on the list. If you prefer a followup,
please let me know.

Thanks again for catching the IB logic detail!

Regards,
David