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

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Message-ID: <aEZkjA1L-dP_Qt3U@infradead.org>
Date: Sun, 8 Jun 2025 21:35:24 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Christian König <christian.koenig@....com>
Cc: wangtao <tao.wangtao@...or.com>, Christoph Hellwig <hch@...radead.org>,
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Subject: Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file_range

On Fri, Jun 06, 2025 at 01:20:48PM +0200, Christian König wrote:
> > dmabuf acts as a driver and shouldn't be handled by VFS, so I made
> > dmabuf implement copy_file_range callbacks to support direct I/O
> > zero-copy. I'm open to both approaches. What's the preference of
> > VFS experts?
> 
> That would probably be illegal. Using the sg_table in the DMA-buf
> implementation turned out to be a mistake.

Two thing here that should not be directly conflated.  Using the
sg_table was a huge mistake, and we should try to move dmabuf to
switch that to a pure dma_addr_t/len array now that the new DMA API
supporting that has been merged.  Is there any chance the dma-buf
maintainers could start to kick this off?  I'm of course happy to
assist.

But that notwithstanding, dma-buf is THE buffer sharing mechanism in
the kernel, and we should promote it instead of reinventing it badly.
And there is a use case for having a fully DMA mapped buffer in the
block layer and I/O path, especially on systems with an IOMMU.
So having an iov_iter backed by a dma-buf would be extremely helpful.
That's mostly lib/iov_iter.c code, not VFS, though.

> The question Christoph raised was rather why is your CPU so slow
> that walking the page tables has a significant overhead compared to
> the actual I/O?

Yes, that's really puzzling and should be addressed first.