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

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

[Karl Mehltretter]

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>
---

Notes:
    Changes in v2:
    - Return -EINVAL when the maximum segment size reported by the device is
      smaller than PAGE_SIZE instead of clamping it. (Christian)
    - Add Jason Gunthorpe's Reviewed-by tag.
    
    Tested on v7.3-rc4-70-gfe2ec83746e5 in QEMU (x86_64, TCG) with
    DMA_API_DEBUG (all_errors=1) and DMABUF_DEBUG, A/B against the same
    base:
    
                                            before        after
      DMA_BUF_IOCTL_SYNC, 64 MiB udmabuf    1 report      0
      vivid import, 4 MiB udmabuf           2 reports     0
      vivid import, 2 MiB hugetlb udmabuf   2 reports     0
        frames captured                     5/5           5/5
    
    vb2-dma-contig rejected the non-contiguous import in both runs.
    
    For v2, a focused importer advertising PAGE_SIZE / 2 returned
    -EINVAL under KASAN and DMA_API_DEBUG. No warning, BUG, or DMA-API
    report was emitted.

 drivers/dma-buf/udmabuf.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index df6dd00462423..018937435356e 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -133,15 +133,22 @@ static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
 {
 	struct udmabuf *ubuf = buf->priv;
 	struct sg_table *sg;
+	unsigned int max_segment;
 	int ret;
 
+	max_segment = dma_get_max_seg_size(dev);
+	/* The SG allocator requires a segment limit of at least PAGE_SIZE. */
+	if (max_segment < PAGE_SIZE)
+		return ERR_PTR(-EINVAL);
+
 	sg = kzalloc_obj(*sg);
 	if (!sg)
 		return ERR_PTR(-ENOMEM);
 
-	ret = sg_alloc_table_from_pages(sg, ubuf->pages, ubuf->pagecount, 0,
-					ubuf->pagecount << PAGE_SHIFT,
-					GFP_KERNEL);
+	ret = sg_alloc_table_from_pages_segment(sg, ubuf->pages, ubuf->pagecount,
+						0, ubuf->pagecount << PAGE_SHIFT,
+						max_segment,
+						GFP_KERNEL);
 	if (ret < 0)
 		goto err_alloc;
 

base-commit: fe2ec83746e501645709761605c2464a44fd2929
-- 
2.53.0

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

[Karl Mehltretter]

On Tue, Sep 29, 2026 at 06:00:27AM +0000, sashiko-bot@kernel.org wrote:
> On architectures with a 256KB PAGE_SIZE (such as Hexagon or PowerPC), this
> new validation evaluates to 65536 < 262144, unconditionally returning
> -EINVAL and breaking all CPU mappings.

Right, the misc device has no dma_parms, so it gets the 64K default and
sync fails with 256K pages

The misc device has no real segment limit, so for v3 I'll give it
dma_parms with an unlimited max segment size and keep the -EINVAL for
importers that report a limit below PAGE_SIZE.

Karl

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

[Jason Gunthorpe]

On Tue, Sep 29, 2026 at 08:56:01AM +0200, Karl Mehltretter wrote:
> On Tue, Sep 29, 2026 at 06:00:27AM +0000, sashiko-bot@kernel.org wrote:
> > On architectures with a 256KB PAGE_SIZE (such as Hexagon or PowerPC), this
> > new validation evaluates to 65536 < 262144, unconditionally returning
> > -EINVAL and breaking all CPU mappings.
> 
> Right, the misc device has no dma_parms, so it gets the 64K default and
> sync fails with 256K pages

I would ignore this, it seems fictional to me, and if someone really
needs to support this rediculous combination it should be done inside
sg_alloc_table_from_pages_segment(:)
 
> The misc device has no real segment limit, so for v3 I'll give it
> dma_parms with an unlimited max segment size and keep the -EINVAL for
> importers that report a limit below PAGE_SIZE.

The params have to come from the *importing* device, not the udmabuf
misc device. You don't get to control what they are, and there is no
reason to put a dma_parms on a misc deivce.

Jason

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

[Christian König]

On 9/29/26 07:48, 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>

Reviewed-by: Christian König <christian.koenig@amd.com>

> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
> ---
> 
> Notes:
>     Changes in v2:
>     - Return -EINVAL when the maximum segment size reported by the device is
>       smaller than PAGE_SIZE instead of clamping it. (Christian)
>     - Add Jason Gunthorpe's Reviewed-by tag.
>     
>     Tested on v7.3-rc4-70-gfe2ec83746e5 in QEMU (x86_64, TCG) with
>     DMA_API_DEBUG (all_errors=1) and DMABUF_DEBUG, A/B against the same
>     base:
>     
>                                             before        after
>       DMA_BUF_IOCTL_SYNC, 64 MiB udmabuf    1 report      0
>       vivid import, 4 MiB udmabuf           2 reports     0
>       vivid import, 2 MiB hugetlb udmabuf   2 reports     0
>         frames captured                     5/5           5/5
>     
>     vb2-dma-contig rejected the non-contiguous import in both runs.
>     
>     For v2, a focused importer advertising PAGE_SIZE / 2 returned
>     -EINVAL under KASAN and DMA_API_DEBUG. No warning, BUG, or DMA-API
>     report was emitted.
> 
>  drivers/dma-buf/udmabuf.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index df6dd00462423..018937435356e 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -133,15 +133,22 @@ static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
>  {
>  	struct udmabuf *ubuf = buf->priv;
>  	struct sg_table *sg;
> +	unsigned int max_segment;
>  	int ret;
>  
> +	max_segment = dma_get_max_seg_size(dev);
> +	/* The SG allocator requires a segment limit of at least PAGE_SIZE. */
> +	if (max_segment < PAGE_SIZE)
> +		return ERR_PTR(-EINVAL);
> +
>  	sg = kzalloc_obj(*sg);
>  	if (!sg)
>  		return ERR_PTR(-ENOMEM);
>  
> -	ret = sg_alloc_table_from_pages(sg, ubuf->pages, ubuf->pagecount, 0,
> -					ubuf->pagecount << PAGE_SHIFT,
> -					GFP_KERNEL);
> +	ret = sg_alloc_table_from_pages_segment(sg, ubuf->pages, ubuf->pagecount,
> +						0, ubuf->pagecount << PAGE_SHIFT,
> +						max_segment,
> +						GFP_KERNEL);
>  	if (ret < 0)
>  		goto err_alloc;
>  
> 
> base-commit: fe2ec83746e501645709761605c2464a44fd2929

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

[Karl Mehltretter]

On Tue, Sep 29, 2026 at 09:03:17AM +0100, Jason Gunthorpe wrote:
> On Tue, Sep 29, 2026 at 08:56:01AM +0200, Karl Mehltretter wrote:
> > Right, the misc device has no dma_parms, so it gets the 64K default and
> > sync fails with 256K pages
> 
> I would ignore this, it seems fictional to me, and if someone really
> needs to support this rediculous combination it should be done inside
> sg_alloc_table_from_pages_segment(:)
>  

Fair enough, Sashiko got me there. Looking around, 256K pages seem
to be barely supported today: they were already disabled for 47x, and
I couldn't get userspace running on a 256K-page 44x kernel in QEMU.

I'll leave v2 as is. Thanks for the review.

Karl