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

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Date:	Wed, 20 Apr 2016 11:59:36 -0400
From:	Waiman Long <waiman.long@....com>
To:	Dave Chinner <david@...morbit.com>
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 04/19/2016 07:01 PM, Dave Chinner wrote:
> On Mon, Apr 18, 2016 at 03:46:46PM -0400, Waiman Long wrote:
>> 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.
>> 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.
> IOWs, you're saying that you won't fix the problem, because all you
> care about is scalability results. This is how we end up with code
> that breaks randomly in future because if it doesn't get fixed now,
> nobody will fix the underlying problem. So, fix it now, fix it
> properly and you still get your scalability improvement without
> leaving a landmine that will explode on someone else in future.
>
> Fix it now, fix it properly.

I am not saying that I will not fix it. I am just saying that I need 
more time to fully understand what code changes need to be done. I am 
not that well versed in the filesystem internal, though it will be a 
good learning experience for me.

Cheers,
Longman