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

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Message-ID: <754b4cc9-20ab-4d87-85bf-eb56be058856@amd.com>
Date: Wed, 7 Jan 2026 16:56:05 +0100
From: Christian König <christian.koenig@....com>
To: Ming Lei <ming.lei@...hat.com>, Christoph Hellwig <hch@....de>
Cc: Pavel Begunkov <asml.silence@...il.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 02:42, 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).

Yeah that would make it much more cleaner.

Also some higher level documentation would certainly help.

> 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.

Yeah that it might be better if it's more nvme specific came to me as well.

Regards,
Christian.

>  
> 
> Thanks, 
> Ming
>