Re: Oops while rebalancing, now unmountable.

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Christoph Hellwig's message of 2010-11-15 14:12:04 -0500:
> On Mon, Nov 15, 2010 at 07:46:57PM +0100, Andrea Arcangeli wrote:
> > I've been reading the writeout() in mm/migrate.c and I wonder if maybe
> > that should have been WB_SYNC_ALL or if we miss a
> > wait_on_page_writeback in after ->writepage() returns? Can you have a
> > look there? We check the PG_writeback bit when the page is not dirty
> > (well before fallback_migrate_page is called), but after calling
> > writeout() we don't return to wait on PG_writeback. We make sure to
> > hold the page lock after ->writepage returns but that doesn't mean
> > PG_writeback isn't still set.
> 
> I didn't even notice that, but the WB_SYNC_NONE does indeed seem
> buggy to me.  If we set the sync_mode to WB_SYNC_NONE filesystem
> can and frequently do trylock operations and might just skip to
> write it out completely.
> 
> So we defintively do need to change writeout to do a WB_SYNC_ALL
> writeback.  In addition to that we'll also need the
> wait_on_page_writeback call to make sure we actually wait for I/O
> to finish.
> 
> Also what protects us from updating the page while we write it out?
> PG_writeback on many filesystems doesn't protect writes from modifying
> the in-flight buffer, and just locking the page after ->writepage
> is racy without a check that nothing changed.
> 

Oh, I should have thought of that.  Btrfs (and most of the time xfs?)
will wait on PageWriteback internally, but for the ext crowd we're in
trouble.

-chris

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