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

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Date:   Tue, 11 Feb 2020 08:38:31 -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 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.

  Remove the check because DAX now has it's own read/write methods and
  file systems which support DAX check IS_DAX() prior to IOCB_DIRECT.
  Therefore, it does not matter if the file state is DAX when the iocb
  flags are created, and we can avoid the race.

  Reviewed-by: Jan Kara <jack@...e.cz>
  Signed-off-by: Ira Weiny <ira.weiny@...el.com>
</commit msg>

Ira

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