linux-kernel - Re: [PATCH block/for-linus] writeback: fix syncing of I_DIRTY

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Date:	Mon, 24 Aug 2015 14:10:38 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Eryu Guan <eguan@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
	xfs@....sgi.com, axboe@...com, Jan Kara <jack@...e.com>,
	linux-fsdevel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH block/for-linus] writeback: fix syncing of I_DIRTY_TIME
 inodes

Hello, Dave.

On Fri, Aug 21, 2015 at 09:04:51AM +1000, Dave Chinner wrote:
> > Maybe I'm misunderstanding the code but all xfs_writepage() calls are
> > from unbound workqueues - the writeback workers - while
> > xfs_setfilesize() are from bound workqueues, so I wondered why that
> > was and looked at the code and the setsize functions are run off of a
> > separate work item which is queued from the end_bio callback and I
> > can't tell who would be waiting for them.  Dave, what am I missing?
> 
> xfs_setfilesize runs transactions, so it can't be run from IO
> completion context as it needs to block (i.e. on log space or inode
> locks). It also can't block log IO completion, nor metadata Io
> completion, as only log IO completion can free log space, and the
> inode lock might be waiting on metadata buffer IO completion (e.g.
> during delayed allocation). Hence we have multiple IO completion
> workqueues to keep these things separated and deadlock free. i.e.
> they all get punted to a workqueue where they are then processed in
> a context that can block safely.

I'm still a bit confused.  What prevents the following from happening?

1. io completion of last dirty page of an inode and work item for
   xfs_setfilesize() is queued.

2. inode removed from dirty list.

3. __sync_filesystem() invokes sync_inodes_sb().  There are no dirty
   pages, so it finishes.

4. xfs_fs_sync_fs() is called which calls _xfs_log_force() but the
   work item from #1 hasn't run yet, so the size update isn't written
   out.

5. Crash.

Is it that _xfs_log_force() waits for the setfilesize transaction
created during writepage?

Thanks.

-- 
tejun
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