Re: [RFC] extending splice for copy offloading

Jan Kara
2013-10-02T12:58:45Z

On Tue 01-10-13 12:58:17, Zach Brown wrote:
> > - app calls splice(from, 0, to, 0, SIZE_MAX)
> > 1) VFS calls ->direct_splice(from, 0, to, 0, SIZE_MAX)
> > 1.a) fs reflinks the whole file in a jiffy and returns the size of the file
> > 1 b) fs does copy offload of, say, 64MB and returns 64M
> > 2) VFS does page copy of, say, 1MB and returns 1MB
> > - app calls splice(from, X, to, X, SIZE_MAX) where X is the new offset
>
> (It's not SIZE_MAX. It's MAX_RW_COUNT. INT_MAX with some
> PAGE_CACHE_SIZE rounding noise. For fear of weird corners of fs code
> paths that still use int, one assumes.)
>
> > The point is: the app is always doing the same (incrementing offset
> > with the return value from splice) and the kernel can decide what is
> > the best size it can service within a single uninterruptible syscall.
> >
> > Wouldn't that work?
>
> It seems like it should, if people are willing to allow splice() to
> return partial counts. Quite a lot of IO syscalls technically do return
> partial counts today if you try to write > MAX_RW_COUNT :).
Yes. Also POSIX says that application must handle such case for read &
write. But in practice programmers are lazy.

> But returning partial counts on the order of a handful of megs that the
> file systems make up as the point of diminishing returns is another
> thing entirely. I can imagine people being anxious about that.
>
> I guess we'll find out!
Return 4 KB once in a while to screw up buggy applications from the
start :-p

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR