linux-kernel - Re: [PATCH v3 1/2] ext4: Pass in DIO_SKIP_DIO_COUNT flag if inode_dio

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Date:	Tue, 26 Apr 2016 12:32:17 -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>,
	Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Toshimitsu Kani <toshi.kani@....com>
Subject: Re: [PATCH v3 1/2] ext4: Pass in DIO_SKIP_DIO_COUNT flag if inode_dio_begin()
 called

On 04/25/2016 07:48 AM, Christoph Hellwig wrote:
> On Thu, Apr 21, 2016 at 02:15:24PM -0400, Waiman Long wrote:
>> On 04/20/2016 04:58 PM, Christoph Hellwig wrote:
>>> FYI, none of the Dax code even needs to ever touch the dio_count,
>>> as dax I/O can't be asynchronous, and we thus don't need it to protect
>>> against truncate.  I'd suggest to remove it and then end_io callback
>> >from the DAX code entirely as a start and then move from there.
>>
>> Yes, it seems like we may not need to change the dio_count in dax_do_io()
>> after all. BTW, what do mean by using end_io callback as a start?
> I mean to remove both the i_dio_count manipulation, and the unessecary
> end_io callback from dax_do_io.

Thanks for the clarification.

Since DAX I/O is always synchronous, the locking done by the caller or 
in the dax_do_Io() for read should be enough to prevent truncation from 
happening at the same time. So we don't need to use i_dio_count for that 
purpose.

However, I have not understood enough of the block IO layer to determine 
if the end_io callback is really redundant. I am not confident enough to 
touch the end_io callback.

Cheers,
Longman