linux-kernel - Re: [RFC PATCH] ext4: Don't release mutex for DAX write

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Message-ID: <5728CAB6.4050101@hpe.com>
Date:	Tue, 3 May 2016 11:58:46 -0400
From:	Waiman Long <waiman.long@....com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	<linux-ext4@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH] ext4: Don't release mutex for DAX write

On 05/03/2016 04:43 AM, Christoph Hellwig wrote:
> As explained in another thread I really think we need to get DAX
> to stop pretending to be direct I/O, which should also take care
> of the locking.  The same issue also exists for ext2 and XFS so it
> needs to be solved at a higher level.

I think the DAX code was in the DIO path because it didn't want to use 
buffer cache at all. Taking DAX out from DIO will mean having a third 
mode of doing I/O which is similar to DIO in certain ways, but not 
exactly the same. There will be a certain amount of code duplication in 
this case. Do we really want to do that?

As for the locking problem, xfs doesn't seem to have issue as it uses a 
rwsem in the xfs inode for synchronization. I haven't looked into the 
ext2 code to see if there is any issue there.

Cheers,
Longman