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

[Christian König]

On 6/3/25 15:19, Christoph Hellwig wrote:
> 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?

It unfortunately does, see the numbers on patch 3 and 4.

I'm not very keen about it either, but I don't see much other way to do this.

> 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.

That would work for me as well. But if splice or copy_file_range is used is not that important to me.

My question is rather if it's ok to call f_op->write_iter() and f_op->read_iter() with pages allocated by alloc_pages(), e.g. where drivers potentially ignore the page count and just re-use pages as they like?

Regards,
Christian.