Re: [patch] fs: fix deadlocks in writeback_if_idle

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 24 Nov 2010 12:03:43 +1100
Nick Piggin <npiggin@kernel.dk> wrote:

> On Tue, Nov 23, 2010 at 01:58:24PM -0500, Chris Mason wrote:
> > > > My original btrfs patch just exported the bdi_ funcs so that btrfs could
> > > > do the above internally.  But Christoph objected, and I think he's
> > > > right.  We should either give everyone a bdi or make sure the writeback
> > > > func kicks only one filesystem.
> > > 
> > > Well it's just kicking the writeback thread, and it will writeback
> > > from that particular sb.
> > 
> > Hmmm?  It will writeback for all the SBs on that bdi.  In the current
> > form that ext4 uses, that gets pretty expensive if you have a bunch of
> > large partitions and you're only running out of space on one of them.
> 
> Right. But if the bdi has writeback in progress (which would be most
> of the time, on a busy filesystem), writeback_if_idle doesn't do
> anything, and it is happy just for the background writeback to
> eventually get around to writing out for us.

That doesn't work if you're running btfs (apparently short for
"busticated filesystem") because the bdi-per-sb thing carefully hid the
information which you're looking for.


We still don't have a fix for this bug yet, it appears, btw.