linux-kernel - Re: linux-next: manual merge of the block tree with the vfs tree

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Date:	Mon, 2 Feb 2015 09:06:08 +0100
From:	Christoph Hellwig <hch@....de>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ming Lei <ming.lei@...onical.com>
Subject: Re: linux-next: manual merge of the block tree with the vfs tree

On Sun, Feb 01, 2015 at 05:56:19AM +0000, Al Viro wrote:
> FWIW, there's an interesting question about the second commit in there -
> what do we want vfs_iter_{read,write}() to do with *iter in case if it
> has hit this:
>         if (ret == -EIOCBQUEUED)
>                 ret = wait_on_sync_kiocb(&kiocb);
> 
> Do we require ->read_iter() and ->write_iter() on sync kiocb to do all
> advancing the iter before returning -EIOCBQUEUED?  What's more, do we
> ever want to have it returned on sync kiocb?  IOW, is there any point
> in having that wait in callers?

See my "[RFC] split struct kiocb" series to sort out that mess.  For
now none of the callers relies on the iov_iter being advances, so until
then we can simply ignore that problem until then.

> I'm not sure if ep_io() and ep_aio_rwtail() + wait for completion are
> eqiuvalent; ep_read/ep_write are very easy to turn into sync side of
> ->read_iter/->write_iter and if that's equivalent to ep_aio_read/ep_aio_write
> on sync kiocb + waiting for completion, we are fine.

They are very similar, and yes thet should be moved to iter version of
the methods.  I actually started that but then ran into problems with
the aio core that needed addressing first.
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