Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait /notify + callback chains

[Stephen C. Tweedie]

Hi,

On Sat, Feb 03, 2001 at 12:28:47PM -0800, Linus Torvalds wrote:
> 
> On Thu, 1 Feb 2001, Stephen C. Tweedie wrote:
> > 
> Neither the read nor the write are page-aligned. I don't know where you
> got that idea. It's obviously not true even in the common case: it depends
> _entirely_ on what the file offsets are, and expecting the offset to be
> zero is just being stupid. It's often _not_ zero. With networking it is in
> fact seldom zero, because the network packets are seldom aligned either in
> size or in location.

The underlying buffer is.  The VFS (and the current kiobuf code) is
already happy about IO happening at odd offsets within a page.
However, the more general case --- doing zero-copy IO on arbitrary
unaligned buffers --- simply won't work if you expect to be able to
push those buffers to disk without a copy.  

The splice case you talked about is fine because it's doing the normal
prepare/commit logic where the underlying buffer is page aligned, even
if the splice IO is not to a page aligned location.  That's _exactly_
what kiobufs were intended to support.  The prepare_read/prepare_write/
pull/push cycle lets the caller tell the pull() function where to
store its data, becausse there are alignment constraints which just
can't be ignored: you simply cannot do physical disk IO on
non-sector-aligned memory or in chunks which aren't a multiple of
sector size.  (The buffer address alignment can sometimes be relaxed
--- obviously if you're doing PIO then it doesn't matter --- but the
length granularity is rigidly enforced.)
 
Cheers,
 Stephen