Re: [RFC] situation with csum_and_copy_... API

From: Al Viro
Date: Fri Nov 21 2014 - 14:39:25 EST

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
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