Re: [PATCH block/for-linus] writeback: fix syncing of I_DIRTY_TIME inodes

From: Dave Chinner
Date: Mon Aug 24 2015 - 18:27:33 EST

On Mon, Aug 24, 2015 at 02:10:38PM -0400, Tejun Heo wrote:
> 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.

The inode has already been removed from the dirty list - that
happens at inode writeback submission time, not IO completion.

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

There are no dirty pages, but the pages aren't clean, either. i.e
they are still under writeback. Hence we need to invoke
wait_inodes_sb() to wait for writeback on all pages to complete
before returning.

> 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.

The bug here is that wait_inodes_sb() has not been run, therefore
->syncfs is being run before IO completions have been processed and
pages marked clean.

> 5. Crash.
> Is it that _xfs_log_force() waits for the setfilesize transaction
> created during writepage?

No, it's wait_inodes_sb() that does the waiting for data IO
completion for sync.


Dave Chinner
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