Re: [RFC] extending splice for copy offloading

Szeredi Miklos
2013-09-20T09:49:59Z

On Wed, Sep 11, 2013 at 7:06 PM, Zach Brown <zab@redhat.com> wrote:
>
> When I first started on this stuff I followed the lead of previous
> work and added a new syscall for the copy operation:
>
> https://lkml.org/lkml/2013/5/14/618
>
> Towards the end of that thread Eric Wong asked why we didn't just
> extend splice. I immediately replied with some dumb dismissive
> answer. Once I sat down and looked at it, though, it does make a
> lot of sense. So good job, Eric. +10 Dummie points for me.
>
> Extending splice avoids all the noise of adding a new syscall and
> naturally falls back to buffered copying as that's what the direct
> splice path does for sendfile() today.

Nice idea.

>
> So that's what this patch series demonstrates. It adds a flag that
> lets splice get at the same direct splicing that sendfile() does.
> We then add a file system file_operations method to accelerate the
> copy which has access to both files.
>
> Some things to talk about:
> - I really don't care about the naming here. If you do, holler.
> - We might want different flags for file-to-file splicing and acceleration

Yes, I think "copy" and "reflink" needs to be differentiated.

> - We might want flags to require or forbid acceleration
> - We might want to provide all these flags to sendfile, too
>
> Thoughts? Objections?

Can filesystem support "whole file copy" only? Or arbitrary
block-to-block copy should be mandatory?

Splice has size_t argument for the size, which is limited to 4G on 32
bit. Won't this be an issue for whole-file-copy? We could have
special value (-1) for whole file, but that's starting to be hackish.

We are talking about copying large amounts of data in a single
syscall, which will possibly take a long time. Will the syscall be
interruptible? Restartable?

Thanks,
Miklos