linux-kernel - Re: [PATCH] block: remove plugging at buffered write time

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Message-ID: <20120210024716.GA12259@localhost>
Date:	Fri, 10 Feb 2012 10:47:16 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Chris Mason <chris.mason@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>, Li Shaohua <shaohua.li@...el.com>
Cc:	Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] block: remove plugging at buffered write time

On Fri, Feb 10, 2012 at 09:52:18AM +0800, Wu Fengguang wrote:
> 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.

It's actually wrong to do plugging around vfs_fsync_range().

Because these call paths

        write() with O_SYNC
          generic_write_sync()
            vfs_fsync_range()
              ->fsync()
              generic_file_fsync()

        fsync()
          do_fsync()
            vfs_fsync()
              vfs_fsync_range()

pass arbitrary @size arguments, which may be much larger than the
preferable I/O size, or may cross extent/device boundaries.

generic_file_fsync() starts with a filemap_write_and_wait_range()
call, which already has proper plugging somewhere underneath. Then
followed by metadata writes, which has plugging inside
fsync_buffers_list(). At last, sync_inode_metadata() calls into
->write_inode() which may or may not care plugging.

The other fs specific ->fsync() do similar steps, varying in the
metadata and fs specific housekeeping part.

I'll just drop this code. Shall the fs specific metadata I/O be
plugged accordingly? I'm afraid this is beyond my knowledge base...

Thanks,
Fengguang
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