Re: [PATCH v2] udmabuf: respect the device's maximum segment size

[Karl Mehltretter <kmehltretter@gmail.com>]

On Tue, Sep 29, 2026 at 07:48:35AM +0100, Karl Mehltretter wrote:
> get_sg_table() merges physically contiguous pages without accounting
> for the mapping device's maximum segment size. This affects both
> importer mappings and the udmabuf misc device mapping used for CPU
> access.
> 
> With DMA_API_DEBUG enabled, DMA_BUF_IOCTL_SYNC on a 64 MiB udmabuf
> reports:
> 
>   DMA-API: misc udmabuf: mapping sg segment longer than device claims to
>   support [len=65884160] [max=65536]
> 
> Use sg_alloc_table_from_pages_segment() with the mapping device's
> maximum segment size. Return -EINVAL if the limit is smaller than
> PAGE_SIZE because the page-based allocator cannot honor it.
> 
> Before commit 5bf888673e0d ("udmabuf: Do not create malformed
> scatterlists"), each entry covered one page.
> 
> Fixes: 5bf888673e0d ("udmabuf: Do not create malformed scatterlists")
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Friendly ping on v2. It has Reviewed-by tags from Jason and Christian.

Thanks,
Karl