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Message-ID: <20160505152230.GA3994@infradead.org>
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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