netdev - Re: [RFC] situation with csum_and_copy

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Message-ID: <20141120222506.GS7996@ZenIV.linux.org.uk>
Date:	Thu, 20 Nov 2014 22:25:06 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, torvalds@...ux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...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 Thu, Nov 20, 2014 at 01:55:42PM -0800, Eric Dumazet wrote:
> On Thu, 2014-11-20 at 21:47 +0000, Al Viro wrote:
> 
> > As far as I can see, these retries on the send side are simply broken -
> > normally we are talking to TCP sockets there and tcp_sendmsg() does *not*
> > modify iovec in normal case. 
> 
> Arg... I sent this morning something doing this (against net-next tree)
> 
> Is it a problem ?

Yes, it is.  You are breaking several _other_ kernel_sendmsg() users.
They are already slightly broken, but that'll make breakage much more
common.

Please, don't - the right thing to do is to have iov_iter in msghdr
(we already have the kernel and userland ones with different types and
we do not assume their layouts to be identical - currently they are,
but it's easy to change), keep iovec constant in all cases and advance
->msg_iter.  Also in all cases.

Note that direct manipulations of what's currently in ->msg_iov are
wrong - all those loops over vector elements, etc., belong in low-level
primitives.  The main missing ones right now are csum_and_copy_{from,to}_iter()
- I have those in local queue, but I'm still trying to get a reasonably
clean mm/iov_iter.c without ridiculous amounts of boilerplating.  A bit more
massage is needed there...

Seriously, take a look at vfs.git#iov_iter-net; it's preparations for the
one that'll introduce ->msg_iter.  Right now that branch has local iov_iter
declared and initialized in several ->sendmsg() and ->recvmsg() instances and
fed to primitives that work with it; after the conversion it'll be in
msg->msg_iter and it will be initialized by sock_sendmsg()/sock_recvmsg().

The tricky part is how to get through that without temporary breaking the
existing sendmsg/recvmsg users in the kernel *and* without a patch size from
hell.  I more or less see how to carve the remaining steps into
reasonably-sized chunks; iscsi is one of the tricky ones and it, AFAICS,
is genuinely broken in mainline and will need fixes that can go into -stable.

And no, your solution doesn't work.  Sorry.  You'll break e.g. smb_send_kvec()
that way.  ceph_tcp_sendmsg() as well, IIRC.

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