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

[Leon Romanovsky <leon@kernel.org>]

On Thu, Sep 03, 2026 at 10:49:38AM -0300, Jason Gunthorpe wrote:
> On Wed, Sep 02, 2026 at 07:07:50PM -0400, David Hu wrote:
> > On Wed, Sep 2, 2026 at 8:08 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > 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?
> > 
> > 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.
> 
> Nope, I'm fine, it just explains why it wasn't seen in other
> tested. You were backporting and mix&matched things. It confirms the
> upstream kernel was fine from the start.

All that time, I wondered why our testing didn't uncover any issues like
this, given that the requirement to support large BARs was raised almost
immediately during testing.

Thanks

> 
> There are other importers besides RDMA, so I still think that this is
> a good change regardless. Having each segment remain page aligned, and
> staying away from ULONG_MAX that might trigger overflows is a friendly
> and robust thing to do for less sophisticated importers.
> 
> Jason
>