Received-SPF: pass (google.com: best guess record for domain of linux-ext4-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Wed, 12 Feb 2020 07:41:07 +1100
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
Message-ID: <20200211204107.GM10776@dread.disaster.area>
References: <20200208193445.27421-1-ira.weiny@intel.com>
 <20200208193445.27421-6-ira.weiny@intel.com>
 <20200211053401.GE10776@dread.disaster.area>
 <20200211163831.GC12866@iweiny-DESK2.sc.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200211163831.GC12866@iweiny-DESK2.sc.intel.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-ext4-owner@vger.kernel.org
Precedence: bulk

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