linux-ext4 - Re: [PATCH 02/11] ext4: Allow unaligned unlocked DAX IO

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Date:   Wed, 2 Nov 2016 07:27:46 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Dave Chinner <david@...morbit.com>, Ted Tso <tytso@....edu>,
        linux-nvdimm@...1.01.org
Subject: Re: [PATCH 02/11] ext4: Allow unaligned unlocked DAX IO

On Tue, Nov 01, 2016 at 10:06:12PM +0100, Jan Kara wrote:
> Currently we don't allow unaligned writes without inode_lock. This is
> because zeroing of partial blocks could cause data corruption for racing
> unaligned writes to the same block. However DAX handles zeroing during
> block allocation and thus zeroing of partial blocks cannot race. Allow
> DAX unaligned IO to run without inode_lock.

DAX writes should always take the inode lock.  Without that you break
the expectations of existing applications that did not specify O_DIRECT.
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