linux-kernel - Re: [PATCH v5 0/2] ext4: Improve parallel I/O performance on NVDIMM

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Message-ID: <20160501172854.GA19601@infradead.org>
Date:	Sun, 1 May 2016 10:28:54 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <willy@...ux.intel.com>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	Dave Chinner <david@...morbit.com>,
	Christoph Hellwig <hch@...radead.org>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Toshimitsu Kani <toshi.kani@....com>
Subject: Re: [PATCH v5 0/2] ext4: Improve parallel I/O performance on NVDIMM

On Fri, Apr 29, 2016 at 12:38:20PM -0400, Waiman Long wrote:
> From my testing, it looked like that parallel overwrites to the same file in
> an ext4 filesystem on DAX can happen in parallel even if their range
> overlaps. It was mainly because the code will drop the i_mutex before the
> write. That means the overlapped blocks can get garbage. I think this is a
> problem, but I am not expert in the ext4 filesystem to say for sure. I would
> like to know your thought on that.

That's another issue with dax I/O pretending to be direct I/O..  Because
it isn't we'll need to synchronize it like buffered I/O and not like
direct I/O in all file systems.