Re: [RFC] extending splice for copy offloading

[David Lang]

On Wed, 2 Oct 2013, Jan Kara wrote:

> 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

or at least have a debugging option early on that does this so people can use it 
to find such buggy apps.

David Lang
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