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

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Message-ID: <a96e327d-3fef-4d08-87e9-c65866223967@gmail.com>
Date: Tue, 6 Jan 2026 19:51:12 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Ming Lei <ming.lei@...hat.com>, Christoph Hellwig <hch@....de>
Cc: Christian König <christian.koenig@....com>,
 linux-block@...r.kernel.org, io-uring@...r.kernel.org,
 Vishal Verma <vishal1.verma@...el.com>, tushar.gohad@...el.com,
 Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>,
 Sagi Grimberg <sagi@...mberg.me>, Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Sumit Semwal <sumit.semwal@...aro.org>, linux-kernel@...r.kernel.org,
 linux-nvme@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [RFC v2 01/11] file: add callback for pre-mapping dmabuf

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