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

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Message-ID: <aVnFnzRYWC_Y5zHg@fedora>
Date: Sun, 4 Jan 2026 09:42:55 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Christoph Hellwig <hch@....de>
Cc: Christian König <christian.koenig@....com>,
	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 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.
 

Thanks, 
Ming