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

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Date:   Wed, 12 Feb 2020 07:41:07 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Ira Weiny <ira.weiny@...el.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 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...

Cheers,

Dave.

-- 
Dave Chinner
david@...morbit.com