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From: Christoph Hellwig <hch@lst.de>
To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@lst.de>,
	xfs@oss.sgi.com, linux-fsdevel@vger.kernel.org,
	linux-nvdimm@ml01.01.org
Subject: Re: xfs: untangle the direct I/O and DAX path, fix DAX locking
Date: Fri, 24 Jun 2016 09:26:12 +0200
Message-ID: <20160624072612.GA22205@lst.de> (raw)
In-Reply-To: <20160623232446.GA12670@dastard>

On Fri, Jun 24, 2016 at 09:24:46AM +1000, Dave Chinner wrote:
> Except we did that *intentionally* - by definition there is no
> cache to bypass with DAX and so all IO is "direct". That, combined
> with the fact that all Linux filesystems except XFS break the POSIX
> exclusive writer rule you are quoting to begin with, it seemed
> pointless to enforce it for DAX....

No file system breaks the exclusive writer rule - most filesystem
don't make writers atomic vs readers.

More importantly every other filesystem (well there only are ext2
and ext4..) exludes DAX writers against other DAX writers.

> So, before taking any patches to change that behaviour in XFS, a
> wider discussion about the policy needs to be had. I don't think
> we should care about POSIX here - if you have an application that
> needs this serialisation, turn off DAX. That's why I made it a
> per-inode inheritable flag and why the mount option will go away
> over time.

Sorry, but this is simply broken - allowing apps to opt-in behavior
(e.g. like we're using O_DIRECT) is always fine.  Requriring
filesystem-specific tuning that has affect outside the app to get
existing documented behavior is not how to design APIs.

Maybe we'll need to opt-in to use DAX for mmap, but giving the same
existing behavior for read and write and avoiding a copy to the pagecache
is an obvious win.

  parent reply index

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-22 15:27 Christoph Hellwig
2016-06-22 15:27 ` [PATCH 1/8] xfs: don't pass ioflags around in the ioctl path Christoph Hellwig
2016-06-22 15:27 ` [PATCH 2/8] xfs: kill ioflags Christoph Hellwig
2016-06-22 15:27 ` [PATCH 3/8] xfs: remove s_maxbytes enforcement in xfs_file_read_iter Christoph Hellwig
2016-06-22 15:27 ` [PATCH 4/8] xfs: split xfs_file_read_iter into buffered and direct I/O helpers Christoph Hellwig
2016-06-22 15:27 ` [PATCH 5/8] xfs: stop using generic_file_read_iter for direct I/O Christoph Hellwig
2016-06-22 15:27 ` [PATCH 6/8] xfs: direct calls in the direct I/O path Christoph Hellwig
2016-06-22 15:27 ` [PATCH 7/8] xfs: split direct I/O and DAX path Christoph Hellwig
2016-09-29  2:53   ` Darrick J. Wong
2016-09-29  8:38     ` aio completions vs file_accessed race, was: " Christoph Hellwig
2016-09-29 20:18       ` Christoph Hellwig
2016-09-29 20:18         ` Christoph Hellwig
2016-09-29 20:33           ` Darrick J. Wong
2016-06-22 15:27 ` [PATCH 8/8] xfs: fix locking for DAX writes Christoph Hellwig
2016-06-23 14:22   ` Boaz Harrosh
2016-06-23 23:24 ` xfs: untangle the direct I/O and DAX path, fix DAX locking Dave Chinner
2016-06-24  1:14   ` Dan Williams
2016-06-24  7:13     ` Dave Chinner
2016-06-24  7:31       ` Christoph Hellwig
2016-06-24  7:26   ` Christoph Hellwig [this message]
2016-06-24 23:00     ` Dave Chinner
2016-06-28 13:10       ` Christoph Hellwig
2016-06-28 13:27         ` Boaz Harrosh
2016-06-28 13:39           ` Christoph Hellwig
2016-06-28 13:56             ` Boaz Harrosh
2016-06-28 15:39               ` Christoph Hellwig
2016-06-29 12:23                 ` Boaz Harrosh

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