linux-kernel - Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file

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Message-ID: <aD72alIxu718uri4@infradead.org>
Date: Tue, 3 Jun 2025 06:19:38 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Christian König <christian.koenig@....com>
Cc: Christoph Hellwig <hch@...radead.org>, wangtao <tao.wangtao@...or.com>,
	sumit.semwal@...aro.org, kraxel@...hat.com,
	vivek.kasireddy@...el.com, viro@...iv.linux.org.uk,
	brauner@...nel.org, hughd@...gle.com, akpm@...ux-foundation.org,
	amir73il@...il.com, benjamin.gaignard@...labora.com,
	Brian.Starkey@....com, jstultz@...gle.com, tjmercier@...gle.com,
	jack@...e.cz, baolin.wang@...ux.alibaba.com,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	bintian.wang@...or.com, yipengxiang@...or.com, liulu.liu@...or.com,
	feng.han@...or.com
Subject: Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file_range

On Tue, Jun 03, 2025 at 03:14:20PM +0200, Christian König wrote:
> On 6/3/25 15:00, Christoph Hellwig wrote:
> > This is a really weird interface.  No one has yet to explain why dmabuf
> > is so special that we can't support direct I/O to it when we can support
> > it to otherwise exotic mappings like PCI P2P ones.
> 
> With udmabuf you can do direct I/O, it's just inefficient to walk the
> page tables for it when you already have an array of all the folios.

Does it matter compared to the I/O in this case?

Either way there has been talk (in case of networking implementations)
that use a dmabuf as a first class container for lower level I/O.
I'd much rather do that than adding odd side interfaces.  I.e. have
a version of splice that doesn't bother with the pipe, but instead
just uses in-kernel direct I/O on one side and dmabuf-provided folios
on the other.