linux-ext4 - Re: [PATCH v3 05/12] fs: remove unneeded IS

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Message-ID: <20200212160428.GD20214@iweiny-DESK2.sc.intel.com>
Date:   Wed, 12 Feb 2020 08:04:29 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 05/12] fs: remove unneeded IS_DAX() check

On Wed, Feb 12, 2020 at 07:41:07AM +1100, Dave Chinner wrote:
> 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@...el.com wrote:
> > > > From: Ira Weiny <ira.weiny@...el.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...

Deleted.
Ira

> 
> Cheers,
> 
> Dave.
> 
> -- 
> Dave Chinner
> david@...morbit.com