From: Waiman Long <waiman.long@hpe.com>
Subject: Re: [PATCH v3 1/2] ext4: Pass in DIO_SKIP_DIO_COUNT flag if inode_dio_begin()
 called
Date: Mon, 18 Apr 2016 15:46:46 -0400
Message-ID: <571539A6.5070401@hpe.com>
References: <1460484775-33359-1-git-send-email-Waiman.Long@hpe.com> <1460484775-33359-2-git-send-email-Waiman.Long@hpe.com> <20160414031634.GJ10643@dastard> <570FC379.7000107@hpe.com> <20160415081757.GK10643@dastard> <57112235.1090201@hpe.com> <20160415221918.GA21184@destitution>
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Cc: Theodore Ts'o <tytso@mit.edu>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	<linux-ext4@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
	Scott J Norton <scott.norton@hpe.com>,
	Douglas Hatch <doug.hatch@hpe.com>,
	Toshimitsu Kani <toshi.kani@hpe.com>
To: Dave Chinner <david@fromorbit.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20160415221918.GA21184@destitution>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: linux-ext4.vger.kernel.org

On 04/15/2016 06:19 PM, Dave Chinner wrote:
> On Fri, Apr 15, 2016 at 01:17:41PM -0400, Waiman Long wrote:
>> On 04/15/2016 04:17 AM, Dave Chinner wrote:
>>> On Thu, Apr 14, 2016 at 12:21:13PM -0400, Waiman Long wrote:
>>>> What the patch does is to eliminate the innermost
>>>> inode_dio_begin/end pair.
>>> Yes, and with that change inode_dio_wait() no longer waits for
>>> AIO+DIO writes on ext4, hence breaking truncate IO barrier
>>> requirements of inode_dio_wait().
>>>
>>> Cheers,
>>>
>>> Dave.
>> You are right and thank for pointing this out to me. I think I focus too
>> much on the dax_do_io() internal and didn't realize that inode_dio_end() can
>> be deferred in __blockdev_direct_IO(). I will update my patch to eliminate
>> the extra inode_dio_begin/end pair only for dax_do_io().
> Even there there is the risk that a future change will break ext4.
> the ext4 code needs fixing first, then you can look at skipping the
> DIO based counting everywhere.
>
> i.e. fix the root cause of the problem, don't hack around it or
> throw band-aids over it.
>
> Cheers,
>
> Dave.

I agree that the ext4 code needs fixing w.r.t. the problem that you 
found. That will take more time and testing. In the mean time, I think 
it is OK to pick the low-hanging fruits that are handled by my patch.

Cheers,
Longman