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

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Message-ID: <20160414031634.GJ10643@dastard>
Date:	Thu, 14 Apr 2016 13:16:34 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Waiman Long <Waiman.Long@....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 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.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com