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From: Dave Chinner <david@fromorbit.com>
To: Ira Weiny <ira.weiny@intel.com>
Cc: linux-kernel@vger.kernel.org, Jan Kara <jack@suse.cz>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Christoph Hellwig <hch@lst.de>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v3 05/12] fs: remove unneeded IS_DAX() check
Date: Wed, 12 Feb 2020 07:41:07 +1100
Message-ID: <20200211204107.GM10776@dread.disaster.area> (raw)
In-Reply-To: <20200211163831.GC12866@iweiny-DESK2.sc.intel.com>

On Tue, Feb 11, 2020 at 08:38:31AM -0800, Ira Weiny wrote:
> On Tue, Feb 11, 2020 at 04:34:01PM +1100, Dave Chinner wrote:
> > On Sat, Feb 08, 2020 at 11:34:38AM -0800, ira.weiny@intel.com wrote:
> > > From: Ira Weiny <ira.weiny@intel.com>
> > > 
> > > The IS_DAX() check in io_is_direct() causes a race between changing the
> > > DAX state and creating the iocb flags.
> > > 
> > > Remove the check because DAX now emulates the page cache API and
> > > therefore it does not matter if the file state is DAX or not when the
> > > iocb flags are created.
> > 
> > This statement is ... weird.
> > 
> > DAX doesn't "emulate" the page cache API at all
> 
> ah...  yea emulate is a bad word here.
> 
> > - it has it's own
> > read/write methods that filesystems call based on the iomap
> > infrastructure (dax_iomap_rw()). i.e. there are 3 different IO paths
> > through the filesystems: the DAX IO path, the direct IO path, and
> > the buffered IO path.
> > 
> > Indeed, it seems like this works a bit by luck: Ext4 and XFS always
> > check IS_DAX(inode) in the read/write_iter methods before checking
> > for IOCB_DIRECT, and hence the IOCB_DIRECT flag is ignored by the
> > filesystems. i.e. when we got rid of the O_DIRECT paths from DAX, we
> > forgot to clean up io_is_direct() and it's only due to the ordering
> > of checks that we went down the DAX path correctly....
> > 
> > That said, the code change is good, but the commit message needs a
> > rewrite.
> 
> How about?
> 
> <commit msg>
>   fs: Remove unneeded IS_DAX() check
>   
>   The IS_DAX() check in io_is_direct() causes a race between changing the
>   DAX state and creating the iocb flags.

This is irrelevant - the check is simply wrong and has been since
~2016 when we moved DAX to use the iomap infrastructure...

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com

  reply index

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-08 19:34 [PATCH v3 00/12] Enable per-file/directory DAX operations V3 ira.weiny
2020-02-08 19:34 ` [PATCH v3 01/12] fs/stat: Define DAX statx attribute ira.weiny
2020-02-08 19:34 ` [PATCH v3 02/12] fs/xfs: Isolate the physical DAX flag from effective ira.weiny
2020-02-08 19:34 ` [PATCH v3 03/12] fs/xfs: Separate functionality of xfs_inode_supports_dax() ira.weiny
2020-02-11  5:47   ` Dave Chinner
2020-02-11 16:13     ` Ira Weiny
2020-02-08 19:34 ` [PATCH v3 04/12] fs/xfs: Clean up DAX support check ira.weiny
2020-02-11  5:57   ` Dave Chinner
2020-02-11 16:28     ` Ira Weiny
2020-02-11 20:38       ` Dave Chinner
2020-02-08 19:34 ` [PATCH v3 05/12] fs: remove unneeded IS_DAX() check ira.weiny
2020-02-11  5:34   ` Dave Chinner
2020-02-11 16:38     ` Ira Weiny
2020-02-11 20:41       ` Dave Chinner [this message]
2020-02-12 16:04         ` Ira Weiny
2020-02-08 19:34 ` [PATCH v3 06/12] fs/xfs: Check if the inode supports DAX under lock ira.weiny
2020-02-11  6:16   ` Dave Chinner
2020-02-11 17:55     ` Ira Weiny
2020-02-11 20:42       ` Dave Chinner
2020-02-12 16:10         ` Ira Weiny
2020-02-08 19:34 ` [PATCH v3 07/12] fs: Add locking for a dynamic DAX state ira.weiny
2020-02-11  8:00   ` Dave Chinner
2020-02-11 20:14     ` Ira Weiny
2020-02-11 20:59       ` Dan Williams
2020-02-11 21:49       ` Dave Chinner
2020-02-12  6:31         ` Darrick J. Wong
2020-02-08 19:34 ` [PATCH v3 08/12] fs/xfs: Clarify lockdep dependency for xfs_isilocked() ira.weiny
2020-02-08 19:34 ` [PATCH v3 09/12] fs/xfs: Add write DAX lock to xfs layer ira.weiny
2020-02-08 19:34 ` [PATCH v3 10/12] fs: Prevent DAX state change if file is mmap'ed ira.weiny
2020-02-08 19:34 ` [PATCH v3 11/12] fs/xfs: Clean up locking in dax invalidate ira.weiny
2020-02-08 19:34 ` [PATCH v3 12/12] fs/xfs: Allow toggle of effective DAX flag ira.weiny
2020-02-10 15:15 ` [PATCH v3 00/12] Enable per-file/directory DAX operations V3 Jeff Moyer
2020-02-11 20:17   ` Ira Weiny
2020-02-12 19:49     ` Jeff Moyer
2020-02-13 19:01       ` Ira Weiny
2020-02-13 19:05         ` Ira Weiny
2020-02-13 19:58           ` Darrick J. Wong
2020-02-13 23:29             ` Ira Weiny
2020-02-14  0:16               ` Dan Williams
2020-02-14 20:06                 ` Ira Weiny
2020-02-14 21:23                   ` Jeff Moyer
2020-02-14 21:58                     ` Ira Weiny
2020-02-14 22:06                       ` Jeff Moyer
2020-02-14 22:58                         ` Jeff Moyer
2020-02-14 23:03                           ` Jeff Moyer
2020-02-18  2:35                           ` Ira Weiny
2020-02-18 14:22                             ` Jeff Moyer
2020-02-18 23:54                               ` Ira Weiny
2020-02-20 16:20                                 ` Ira Weiny
2020-02-20 16:30                                   ` Darrick J. Wong
2020-02-20 16:49                                     ` Ira Weiny
2020-02-20 17:00                                       ` Darrick J. Wong

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