netdev - RE: [RFC] situation with csum_and_copy

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1C9F6930@AcuExch.aculab.com>
Date:	Fri, 21 Nov 2014 17:42:55 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Al Viro' <viro@...IV.linux.org.uk>,
	Eric Dumazet <eric.dumazet@...il.com>
CC:	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
...
> We would be better off with iov_iter passed to __sock_{send,recv}msg() (as
> a part of struct msghdr, instead of ->msg_iov/->msg_iovlen) and always
> advanced to match the amount of data actually picked from it.  With iovec
> behind it remaining constant.  That would work just as well as the current
> variant for sendmsg(2)/recvmsg(2)/etc., be a lot more convenient for
> kernel_{send,recv}msg() callers and would allow a lot of other fun stuff.

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.

So it would be useful if the iterator could be initialised to a byte
offset down the iov[].

Are there any current code paths where the iov[] is modified but
ends up being something other than 'the remaining data'?
If not then code can check whether iov[0].len has changed, and
skip the 'advance' is it has.

(I've an out-of-tree driver that assumes the iov[] isn't changed.)

	David



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