[RFC v2 01/11] file: add callback for pre-mapping dmabuf

[Pavel Begunkov]

On 1/4/26 01:42, Ming Lei wrote:
> On Thu, Dec 04, 2025 at 02:10:25PM +0100, Christoph Hellwig wrote:
>> On Thu, Dec 04, 2025 at 12:09:46PM +0100, Christian König wrote:
>>>> I find the naming pretty confusing a well.  But what this does is to
>>>> tell the file system/driver that it should expect a future
>>>> read_iter/write_iter operation that takes data from / puts data into
>>>> the dmabuf passed to this operation.
>>>
>>> That explanation makes much more sense.
>>>
>>> The remaining question is why does the underlying file system / driver
>>> needs to know that it will get addresses from a DMA-buf?
>>
>> This eventually ends up calling dma_buf_dynamic_attach and provides
>> a way to find the dma_buf_attachment later in the I/O path.
> 
> Maybe it can be named as ->dma_buf_attach()?  For wiring dma-buf and the
> importer side(nvme).
> 
> But I am wondering why not make it as one subsystem interface, such as nvme
> ioctl, then the whole implementation can be simplified a lot. It is reasonable
> because subsystem is exactly the side for consuming/importing the dma-buf.

It's not an nvme specific interface, and so a file op was much more
convenient. And ioctls for registering it into io_uring would also be
problematic. I simplified some of the layering for the next version,
but most of the complexity comes from handling in blk-mq-dma-token.h,
it'd be same even if made nvme specific. In fact, I had it all first
in nvme but then had to move to block/ because of sleeping.

-- 
Pavel Begunkov