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

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Date:   Tue, 11 Feb 2020 16:34:01 +1100
From:   Dave Chinner <david@...morbit.com>
To:     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 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 - 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.

Cheers,

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