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.
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
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
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