netdev - Re: getting rid of ->splice

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Message-ID: <20141105184945.GS7996@ZenIV.linux.org.uk>
Date:	Wed, 5 Nov 2014 18:49:45 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>, linux-fsdevel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: getting rid of ->splice_write?

On Mon, Sep 22, 2014 at 10:30:53AM -0700, Christoph Hellwig wrote:
> Currently only /dev/null, fusedev and the socket code have a
> splice_write implementation that isn't iter_file_splice_write, and
> it seems like these three could easily be switched over if they
> implemented a ->write_iter.

Not really.  A minor nitpick is that you've missed port_fops_splice_write(),
but the real bitch isn't that and not even the sockets (see the fun with
iov_iter sendmsg/recvmsg work getting resurrected).  It's that NULL
->splice_write() means default_file_splice_write.  IOW, you'd need either
->write_iter() for _everything_ (with support of bvec-backed ones included,
since that's what iter_file_splice_write() will feed to ->write_iter()),
or you need to have do_splice_from() check if ->write_iter is NULL and
go for default_file_splice_write() instead of iter_file_splice_write().

The latter might be doable, but the former is really over the top - for
that we'd need to touch every driver instance of ->write() out there.
You want to do that, it's your funeral...

> Similarly it seems to be like we could kill ->splice_read by
> implementing an equivalent iteration over ->read_iter.

Hard to do.  I agree that we want to, but it'll take quite a bit of work
on iov_iter primitives, I'm afraid.  At the very least, we want a variant
of iov_iter that could steal pages.  Don't forget that a large part of
the rationale behind splice_read was the ability to play zero-copy games.

I'm not sure if it will happen this cycle; there's more than enough fun
on the net/* side.  _If_ that ends up done faster than I expect it to be,
->splice_read() is the obvious next target.
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