[dm-devel] Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex

[Pavel Machek <pavel ucw cz>]

Hi!

> > > This means, however, that we can leave the patch as is (well, with the minor
> > > fix I have already posted), for now, because it doesn't make things worse a
> > > bit, but:
> > > (a) it prevents xfs from being corrupted and
> > 
> > I'd really prefer it to be fixed by 'freezeable workqueues'.
> 
> I'd prefer that you just freeze the filesystem and let the
> filesystem do things correctly.

Well, I'd prefer filesystems not to know about suspend, and current
"freeze the filesystem" does not really nest properly.

> > Can you
> > point me into sources -- which xfs workqueues are problematic?
> 
> AFAIK, its the I/O completion workqueues that are causing problems.
> (fs/xfs/linux-2.6/xfs_buf.c) However, thinking about it, I'm not
> sure that the work queues being left unfrozen is the real problem.
> 
> i.e. after a sync there's still I/O outstanding (e.g. metadata in
> the log but not on disk), and because the kernel threads are frozen
> some time after the sync, we could have issued this delayed write
> metadata to disk after the sync. With XFS, we can have a of queue of

That's okay, snapshot is atomic. As long as data are safely in the
journal, we should be okay.

> However, even if you stop the workqueue processing, you're still
> going to have to wait for all I/O completion to occur before
> snapshotting memory because having any I/O complete changes memory
> state.  Hence I fail to see how freezing the workqueues really helps
> at all here....

It is okay to change memory state, just on disk state may not change
after atomic snapshot.
									Pavel
-- 
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