Date: Fri, 10 Feb 2012 09:52:18 +0800
From: Wu Fengguang <fengguang.wu@intel.com>
To: Chris Mason <chris.mason@oracle.com>,
        Christoph Hellwig <hch@infradead.org>,
        Dave Chinner <david@fromorbit.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Jens Axboe <axboe@kernel.dk>, Li Shaohua <shaohua.li@intel.com>
Subject: Re: [PATCH] block: remove plugging at buffered write time
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On Thu, Feb 09, 2012 at 01:30:27PM -0500, Chris Mason wrote:
> On Thu, Feb 09, 2012 at 01:06:35PM -0500, Christoph Hellwig wrote:
> > On Thu, Feb 09, 2012 at 04:02:24PM +0800, Wu Fengguang wrote:
> > > On Thu, Feb 09, 2012 at 10:27:19AM +1100, Dave Chinner wrote:
> > > > On Wed, Feb 08, 2012 at 07:01:44PM +0800, Wu Fengguang wrote:
> > > > > Buffered write(2) is not directly tied to IO, so it's not suitable to
> > > > > handle plug in generic_file_aio_write().
> > > > 
> > > > But generic_sync_write() does issue IO for O_SYNC writes, so unless
> > > > there is plugging at a lower layer in the writeback code then it
> > > > appears to me that plugging is still necessary (at least inside the
> > > > sync branch)....
> > > 
> > > Good catch! It looks that generic_write_sync() eventually calls into
> > > vfs_fsync_range() which further calls ->fsync(). We may add plugging
> > > around it:
> > 
> > 
> > NAK, please keep the plugging down in the fs, or the libraries used but
> > not common VFS code.
> 
> Please, what Christoph said.  At least for btrfs plugging here is wrong.

OK, I get the point: the fs knows best when to unplug. Since any
higher level plug nesting will turn such low level efforts into no-op,
it's highly undesirable to do it in the high level.

Thanks,
Fengguang
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