netdev - RE: [RFC] situation with csum_and_copy

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Date:	Mon, 24 Nov 2014 10:03:05 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Al Viro' <viro@...IV.linux.org.uk>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"target-devel@...r.kernel.org" <target-devel@...r.kernel.org>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: RE: [RFC] situation with csum_and_copy_... API

From: Al Viro
> On Fri, Nov 21, 2014 at 05:42:55PM +0000, David Laight wrote:
> 
> > Callers of kernel_send/recvmsg() could easily be using a wrapper
> > function that creates the 'msghdr'.
> > When the want to send the remaining part of a buffer the old iterator
> > will no longer be available - just the original iov and the required offset.
> 
> Er...  So why not copy a struct iov_iter to/from msg->msg_iter, then?
> It's not as it had been particulary large - 5 words isn't much...
> 
> I'm not at all sure that _anything_ has valid reasons for draining iovecs.
> Maintaining a struct iov_iter and modifying it is easy and actually faster...
> 
> Right now the main examples outside of net/* are due to unfortunate
> limitations of ->sendmsg() - until now it had no way to be told that
> desired data starts at offset.  With ->msg_iter it obviously becomes
> possible...

It may well be easier for code that only has to run in a new kernel.
But for code that has run in old kernels as well you still need
to modify the iov[].

This is also true of userspace - there is no way of completing
a partial transfer without modifying the iov[] to allow for the
partial transfer.

Note that I'm not suggesting that any of the 'write' functions
should ever modify an iov[].

	David



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