On Wed 24-11-10 12:03:43, Nick Piggin wrote:
> > For the _nr variant that btrfs uses, it's worse for the filesystems
> > that don't have a 1:1 bdi<->sb mapping. It might not actually write any
> > of the pages from the SB that is out of space.
>
> That's true, but it might not write anything anyway (and after we
> check whether writeout is in progress, the writeout thread could go
> to sleep and not do anything anyway).
>
> So it's a pretty hacky interface anyway. If you want to do anything
> deterministic, you obviously need real coupling between producer and
> consumer. This should only be a performance tweak (or a workaround
> hack in worst case).
Yes, the current interface is a band aid for the problem and better
interface is welcome. But it's not trivial to do better...
> > > It makes no further guarantees, and anyway
> > > the sb has to compete for normal writeback within this bdi.
> >
> > >
> > > I think Christoph is right because filesystems should not really
> > > know about how bdi writeback queueing works. But I don't know if it's
> > > worth doing anything more complex for this functionality?
> >
> > I think we should make a writeback_inodes_sb_unlocked() that doesn't
> > warn when the semaphore isn't held. That should be enough given where
> > btrfs and ext4 are calling it from.
>
> It doesn't solve the bugs -- calling and waiting for writeback is
> still broken because completion requires i_mutex and it is called
> from under i_mutex.
Well, as I wrote in my previous email, only ext4 has the problem with
i_mutex and I personally view it as a bug. But ultimately it's Ted's call
to decide.
Honza
--
Jan Kara <[email protected]>
SUSE Labs, CR
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