linux-ext4 - Re: [RFC][PATCH 8/9 v1] ext4: refine unwritten extent conversion

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Date:	Mon, 31 Dec 2012 18:04:36 +0100
From:	Jan Kara <jack@...e.cz>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [RFC][PATCH 8/9 v1] ext4: refine unwritten extent conversion

On Mon 31-12-12 17:36:21, Jan Kara wrote:
> On Mon 24-12-12 15:55:41, Zheng Liu wrote:
> > From: Zheng Liu <wenqing.lz@...bao.com>
> > 
> > Currently all unwritten extent conversion work is pushed into a workqueue to be
> > done because DIO end_io is in a irq context and this conversion needs to take
> > i_data_sem locking.  But we couldn't take a semaphore in a irq context.  After
> > tracking all extent status, we can first convert this unwritten extent in extent
> > status tree, and call aio_complete() and inode_dio_done() to notify upper level
> > that this dio has done.  We don't need to be worried about exposing a stale data
> > because we first try to lookup in extent status tree.  So it makes us to see the
> > latest extent status.  Meanwhile we queue a work to convert this unwritten
> > extent in extent tree.  After this improvement, reader also needn't wait this
> > conversion to be done when dioread_nolock is enabled.
> > 
> > CC: Jan Kara <jack@...e.cz>
> > CC: "Darrick J. Wong" <darrick.wong@...cle.com>
> > CC: Christoph Hellwig <hch@...radead.org>
> > Signed-off-by: Zheng Liu <wenqing.lz@...bao.com>
> > ---
> > [Cc' to Jan, Darrick, and Christoph because Christoph is trying to handle
> > O_(D)SYNC for AIO]
> > 
> > Hi Jan, Darrick, and Christoph,
> > 
> > This patch refines the unwritten extent conversion in ext4.  Now we can call
> > aio_complete() and inode_dio_done() in end_io function.  I believe Christoph's
> > patch also can work well after applied this patch.  Could you please review
> > this patch?
>   Umm, I don't understand one thing (please bear with me, I've not followed
> extent status tree work in detail): After you report IO completion, you
> must make sure subsequent read returns data you wrote. Thus you would need
> to track also physical location of all extents that are written but not yet
> converted in the extent status tree. I'm not sure which patches are in
> flight but it's definitely not happening right now and it seems to me it
> would complicate the extent status tree (effectively making a full extent
> cache out of it, making it considerably heavier etc.). If extent tree is
> really going that way, then what you propose is probably a good idea. I'm
> still somewhat uneasy about completing the IO before it's really on disk
> (we still need flushing of conversions in various places) but doing the
> conversion before completing the IO has its own (locking) issues especially
> for writeback path. So the solution using extent status tree is fine.
  Ah, the patch is part of a series which changes the extent tree :) OK,
I'm looking into the patches...

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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