linux-ext4 - Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax

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Date:	Thu, 5 May 2016 08:22:30 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Boaz Harrosh <boaz@...xistor.com>, linux-block@...r.kernel.org,
	linux-ext4 <linux-ext4@...r.kernel.org>, Jan Kara <jack@...e.cz>,
	Matthew Wilcox <matthew@....cx>,
	Dave Chinner <david@...morbit.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	XFS Developers <xfs@....sgi.com>, Jens Axboe <axboe@...com>,
	Linux MM <linux-mm@...ck.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-nvdimm <linux-nvdimm@...1.01.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

On Thu, May 05, 2016 at 08:15:32AM -0700, Dan Williams wrote:
> > Agreed - makig O_DIRECT less direct than not having it is plain stupid,
> > and I somehow missed this initially.
> 
> Of course I disagree because like Dave argues in the msync case we
> should do the correct thing first and make it fast later, but also
> like Dave this arguing in circles is getting tiresome.

We should do the right thing first, and make it fast later.  But this
proposal is not getting it right - it still does not handle errors
for the fast path, but magically makes it work for direct I/O by
in general using a less optional path for O_DIRECT.  It's getting the
worst of all choices.

As far as I can tell the only sensible option is to:

 - always try dax-like I/O first
 - have a custom get_user_pages + rw_bytes fallback handles bad blocks
   when hitting EIO

And then we need to sort out the concurrent write synchronization.
Again there I think we absolutely have to obey Posix for the !O_DIRECT
case and can avoid it for O_DIRECT, similar to the existing non-DAX
semantics.  If we want any special additional semantics we _will_ need
a special O_DAX flag.
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