Re: [PATCH] mm/hmm: Allow hmm_dma_map_alloc() to tolerate NULL device

[Daisuke Matsuda <dskmtsd@gmail.com>]

On 2025/05/23 23:48, Christoph Hellwig wrote:
> On Fri, May 23, 2025 at 02:35:37PM +0000, Daisuke Matsuda wrote:
>> Some drivers (such as rxe) may legitimately call hmm_dma_map_alloc() with a
>> NULL device pointer,
> 
> No, they may not.  If something has no device with physical DMA
> capabilities, it has not business calling into it.
> 

Hi Christoph,

RXE is a software emulator of IBTA RoCEv2, designed to allow systems equipped with standard Ethernet adapters to interoperate with other RoCEv2-capable nodes.

Like other Infiniband subsystem drivers (under drivers/infiniband/{hw,sw}), RXE depends on the ib_core and ib_uverbs layers in drivers/infiniband/core. These common RDMA layers, in turn, rely on the HMM infrastructure for specific features such as On-Demand Paging.

As a result, even though RXE lacks physical DMA capabilities, it still needs to interact with hmm_dma_map_alloc() through the shared RDMA core paths. This patch ensures that such software-only use cases do not trigger unintended null pointer dereferences.

Thanks,
Daisuke