Re: [PATCH v4 0/4] Implement dmabuf direct I/O via copy_file_range

["Christian König" <christian.koenig@amd.com>]

On 6/6/25 11:52, wangtao wrote:
> 
> 
>> -----Original Message-----
>> From: Christoph Hellwig <hch@infradead.org>
>> Sent: Tuesday, June 3, 2025 9:20 PM
>> To: Christian König <christian.koenig@amd.com>
>> Cc: Christoph Hellwig <hch@infradead.org>; wangtao
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>> 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.
> If the VFS layer recognizes dmabuf type and acquires its sg_table
> and folios, zero-copy could also be achieved. I initially thought
> 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.

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?

Regards,
Christian.

> 
> Regards,
> Wangtao.
>