Re: [PATCH v8 0/2] dma-buf: Fix silent overflow and alignment

[Christian König]

On 9/2/26 11:53, Leon Romanovsky wrote:
> On Wed, Sep 02, 2026 at 10:44:59AM +0200, Christian König wrote:
>> On 9/2/26 10:32, Leon Romanovsky wrote:
>>> On Wed, Sep 02, 2026 at 09:56:06AM +0200, Christian König wrote:
>>>> On 9/2/26 09:39, Leon Romanovsky wrote:
>>>>> On Wed, Sep 02, 2026 at 09:00:46AM +0200, Christian König wrote:
>>>>>> On 9/1/26 19:08, David Hu wrote:
>>>>>>> From: David Hu <xuehaohu@xxxxxxxxxx>
>>>>>>>
>>>>>>> This series address two related issues in scatter-gather mapping,
>>>>>>> specifically for the MMIO based dma-buf mapping. The fixes ensure
>>>>>>> sgt mapping is correct, and proper for large MMIO regions.
>>>>>>>
>>>>>>> Patch 1 fixes a silent integer overflow for mapping length exceeding 4G
>>>>>>> (Previously submitted as [PATCH v7] dma-buf: Fix silent overflow for
>>>>>>> phys vec to sgt)
>>>>>>> https://lore.kernel.org/all/20260609164047.486227-1-xuehaohu@xxxxxxxxxx/
>>>>>>>
>>>>>>> Patch 2 Splits sgl by largest page aligned chunk
>>>>>>> (Previously submitted as [PATCH v3] dma-buf: Split sgl by largest page-aligned chunk)
>>>>>>> https://lore.kernel.org/all/20260722233806.3922093-1-dhu@xxxxxx/
>>>>>>
>>>>>> *sigh* such issues are exactly the reason why I didn't wanted the dma-mapping stuff inside DMA-buf. That clearly doesn't belong here.
>>>>>
>>>>> And this is why so many in the kernel community want to get rid of SG
>>>>> lists. It would be great if DMA-BUF could also eliminate the need to
>>>>> convert to an SGL, like Jason proposed.
>>>>>
>>>>> The DMA layer no longer needs SGL. These bugs belong to the DMA-BUF layer,
>>>>> which is the one that depends on it.
>>>>
>>>> I'm all fine using an array/xarray of dma_addr_t in DMA-buf, just phys_vec is a clear no-go.
>>>
>>> You are proposing the same thing as an SGL, just in a different format.
>>
>> Yes, because that is the right thing todo as far as I can see.
>>
>>> It does not address the issue that dma_addr_t is expected to hold a DMA
>>> address, while that is not always the case. For example, in the P2P case,
>>> the addresses are not DMA addresses.
>>
>> Yes they are. They must be DMA addresses because that is the only thing the importer needs to do it's DMA.
> 
> They can perform DMA, but that still does not make them suitable for the
> dma_addr_t type. For the PCI_P2PDMA_MAP_BUS_ADDR flow, these addresses
> follow completely different rules: they are not unmapped, require no cache
> synchronization, are valid only for peer access, and require separate error
> handling.

The PCI_P2PDMA_MAP_BUS_ADDR is not supported by DMA-buf and as far as I can see is a complete dead end.

> All of this information is lost if only the dma_addr_t is stored.

Yes and that is fully intentional.

DMA-buf handles that cleanly on the buffer object level and not like PCI_P2PDMA_MAP_BUS_ADDR as a completely broken design on a per address/page basis.

Technical background is that the PCI_P2PDMA_MAP_BUS_ADDR approach can only be handled by a very very small subset of HW.

Regards,
Christian.

> 
>>
>> It can be that those are DMA addresses on private interconnects between devices, but it should *never* be a phys_addr_t because that is limited to the address space the CPU can see.
>>
>>> Jason's proposal:
>>> https://lore.kernel.org/all/0-v1-b5cab63049c0+191af-dmabuf_map_type_jgg@xxxxxxxxxx/
>>
>> Yeah, I have commented quite a bit on that.
> 
> Right, I posted it for reference.
> 
> Thanks
> 
>>
>> Regards,
>> Christian.
>>
>>>
>>> Thanks
>>>
>>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>>
>>>>> Thanks
>>>>
>>>>
>>
>>