netdev - Re: [RFC] situation with csum_and_copy

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Message-ID: <20141121193911.GU7996@ZenIV.linux.org.uk>
Date:	Fri, 21 Nov 2014 19:39:11 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	David Laight <David.Laight@...LAB.COM>
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

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...
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