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

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Date:	Fri, 15 Apr 2016 13:17:41 -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 04/15/2016 04:17 AM, Dave Chinner wrote:
> On Thu, Apr 14, 2016 at 12:21:13PM -0400, Waiman Long wrote:
>> On 04/13/2016 11:16 PM, Dave Chinner wrote:
>>> On Tue, Apr 12, 2016 at 02:12:54PM -0400, Waiman Long wrote:
>>>> When performing direct I/O, the current ext4 code does
>>>> not pass in the DIO_SKIP_DIO_COUNT flag to dax_do_io() or
>>>> __blockdev_direct_IO() when inode_dio_begin() has, in fact, been
>>>> called. This causes dax_do_io()/__blockdev_direct_IO() to invoke
>>>> inode_dio_begin()/inode_dio_end() internally.  This doubling of
>>>> inode_dio_begin()/inode_dio_end() calls are wasteful.
>>>>
>>>> This patch removes the extra internal inode_dio_begin()/inode_dio_end()
>>>> calls when those calls are being issued by the caller directly. For
>>>> really fast storage systems like NVDIMM, the removal of the extra
>>>> inode_dio_begin()/inode_dio_end() can give a meaningful boost to
>>>> I/O performance.
>>> Doesn't this break truncate IO serialisation?
>>>
>>> i.e. it appears to me that the ext4 use of inode_dio_begin()/
>>> inode_dio_end() does not cover AIO, where the IO is still in flight
>>> when submission returns. i.e. the inode_dio_end() call
>>> needs to be in IO completion, not in the submitter context. The only
>>> reason it doesn't break right now is that the duplicate accounting
>>> in the DIO code is correct w.r.t. AIO. Hence bypassing the DIO
>>> accounting will cause AIO writes to race with truncate.
>>>
>>> Same AIO vs truncate problem occurs with the indirect read case you
>>> modified to skip the direct IO layer accounting.
>> I don't quite understand how the duplicate accounting is correct wrt
>> AIO. Both the direct and indirect paths are something like:
>>
>>      inode_dio_begin()
>>      ...
>>          inode_dio_begin()
>>          ...
>>          inode_dio_end()
>>      ...
>>      inode_dio_end()
> With AIO:
>
> 	inode_dio_begin()
> 	...
> 		inode_dio_begin()
> 		<submit IO, no wait>
> 	...
> 	inode_dio_end()
> <ext4 returns to userspace with AIO+DIO in progress>
>
> <some time later DIO completes>
> 	dio_complete
> 		  inode_dio_end()
>
> IOWs, the ext4 accounting is broken w.r.t. AIO, where IO submission
> does not wait for IO completion before returning.
>
>> 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().

Cheers,
Longman
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