Re: [PATCH 2/7] RDMA/hfi1: don't pass bogus GFP_ flags to dma_alloc_coherent

Dean Luick
2022-11-16T17:50:09Z

On 11/16/2022 9:45 AM, Christoph Hellwig wrote:
> On Wed, Nov 16, 2022 at 03:15:10PM +0000, Robin Murphy wrote:
>> Coherent DMA buffers are allocated by a kernel driver or subsystem for the
>> use of a device managed by that driver or subsystem, and thus they
>> fundamentally belong to the kernel as proxy for the device. Any coherent
>> DMA buffer may be mapped to userspace with the dma_mmap_*() interfaces, but
>> they're never a "userspace allocation" in that sense.
>
> Exactly. I could not find a place to map the buffers to userspace,
> so if it does that without using the proper interfaces we need to fix
> that as well. Dean, can you point me to the mmap code?

See hfi1_file_mmap(), cases RCV_HDRQ and RCV_EGRBUF, for the two items you changed in hfi1. Both directly use remap_pfn_range(), which is probably the original approved call, but now is now buried deep within dma_mmap_*(). As you say - these should be updated. That said, the eager buffer mapping will stitch together multiple eager buffers into a single user map/vma. I don't see how to do that with the dma_mmap_*() interface.

-Dean
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