linux-ext4 - Re: [PATCH,RFC 7/7] ext4: move ext4_journal_start/stop to mpage_da_map_and

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Date:	Fri, 18 Feb 2011 19:44:04 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	"Ted Ts'o" <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH,RFC 7/7] ext4: move ext4_journal_start/stop to mpage_da_map_and_submit()

On Fri, Feb 18, 2011 at 6:42 PM, Amir Goldstein <amir73il@...il.com> wrote:
>
> On Fri, Feb 18, 2011 at 6:23 AM, Ted Ts'o <tytso@....edu> wrote:
> > On Sat, Feb 12, 2011 at 07:15:57PM -0500, Theodore Ts'o wrote:
> >> Previously, ext4_da_writepages() was responsible for calling
> >> ext4_journal_start() and ext4_journal_stop().  If the blocks had
> >> already been allocated (we don't support journal=data in
> >> ext4_da_writepages), then there's no need to start a new journal
> >> handle.
> >>
> >> By moving ext4_journal_start/stop calls to mpage_da_map_and_submit()
> >> we should significantly reduce the cpu usage (and cache line bouncing)
> >> if the journal is enabled.  This should (hopefully!) be especially
> >> noticeable on large SMP systems.
> >>
> >> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> >
> > Argh, it turns out this doesn't work.  I was getting sporadic
> > deadlocks and I finally figured out the problem.  If a process is
> > holding page locks, it can't call ext4_journal_start() safely in
> > data=ordered, since there's a chance that there won't be enough
> > transaction credits and a new transaction will be started.  And at
> > that point, in data=ordered mode, we may end up calling
> > journal_submit_inode_data_buffers(), which could try to write back the
> > inode pages in question --- which are already locked.
> >
> > This means that we need to start the journal handle long before we
> > know whether or not we really need it.  Boo, hiss!
> >
> > The only way to solve this problem is to do what I've been planning
> > all for a while, which is to add support in ext4_map_blocks() for a
> > mode where it will allocate a region of blocks, but *not* update the
> > extent map.  It will have to store the allocation in an in-memory
> > cache, so that if other CPU's try to request a logical block, it will
> > get the right answer.  However, the actual on-disk extent map can't be
> > updated until *after* the data is safely written on disk (and the
> > pages can thus be unlocked).
> >
> > Once we do that, we'll also be able to ditch ordered mode for good,
> > since it means that there won't be any chance of stale data being
> > revealed, without any of performance disasters involved with
> > data=ordered mode.
> >
> > I have no idea what these changes will do to Amir's snapshot plans,
> > but sorry, getting this right is going to be higher priority.
>
> If anything, memory-only data allocations would be a great contribution
> to extent data move-on-write :-)
>
> It would allow me to split the extent in-memory and defer the decision,
> whether to split the extent on-disk or wait for copy-on-write to complete,
> to data writeback time.
>
> By that time, async copy-on-write sequence may have already completed
> and fragmentation can be avoided.
>
> If you are looking for someone to execute your plan, or write some
> experimental code, I think that Yongqiang would be up for the task
> (hope that's OK with Yongqiang)
No problem with me.
>
> >
> > I may end up submitting the rest of this patch series without this
> > last patch, since it does clean up the code paths a lot, and it should
> > result in a few small performance improvements --- the big performance
> > improvement, found in this patch, we'll have to skip until we can fix
> > up the writeback submission.
> >
> >                                       - Ted
> > --
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> >



--
Best Wishes
Yongqiang Yang
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