Date:	Wed, 24 Nov 2010 12:03:43 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Eric Sandeen <sandeen@...hat.com>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [patch] fs: fix deadlocks in writeback_if_idle

On Tue, Nov 23, 2010 at 01:58:24PM -0500, Chris Mason wrote:
> Excerpts from Nick Piggin's message of 2010-11-23 07:52:23 -0500:
> > On Tue, Nov 23, 2010 at 07:34:07AM -0500, Chris Mason wrote:
> > > Excerpts from Nick Piggin's message of 2010-11-23 05:02:39 -0500:
> > > 
> > > [ ... ]
> > > 
> > > > 
> > > > Avoid both these issues by issuing completely asynchronous writeback request in
> > > > writeback_inodes_sb_if_idle. Don't let that fool you into thinking these
> > > > functions don't suck any more.
> > > > 
> > > > ext4 now passes extensive stress testing with xfstests, fs_mark, dbench,
> > > > with a writeback_inodes_if_idle call added directly into ext4_da_write_begin
> > > > to trigger the path frequently. Previously it would spew lockdep stuff and
> > > > hang in a number of ways very quickly.
> > > > 
> > > > Signed-off-by: Nick Piggin <npiggin@...nel.dk>
> > > > 
> > > > ---
> > > >  fs/fs-writeback.c |   32 ++++++++++++++++++++------------
> > > >  1 file changed, 20 insertions(+), 12 deletions(-)
> > > > 
> > > > Index: linux-2.6/fs/fs-writeback.c
> > > > ===================================================================
> > > > --- linux-2.6.orig/fs/fs-writeback.c    2010-11-23 20:57:23.000000000 +1100
> > > > +++ linux-2.6/fs/fs-writeback.c    2010-11-23 20:59:10.000000000 +1100
> > > > @@ -1152,16 +1152,17 @@ EXPORT_SYMBOL(writeback_inodes_sb);
> > > >   *
> > > >   * Invoke writeback_inodes_sb if no writeback is currently underway.
> > > >   * Returns 1 if writeback was started, 0 if not.
> > > > + *
> > > > + * Even if 1 is returned, writeback may not be started if memory allocation
> > > > + * fails. This function makes no guarantees about anything.
> > > >   */
> > > >  int writeback_inodes_sb_if_idle(struct super_block *sb)
> > > >  {
> > > >      if (!writeback_in_progress(sb->s_bdi)) {
> > > > -        down_read(&sb->s_umount);
> > > > -        writeback_inodes_sb(sb);
> > > > -        up_read(&sb->s_umount);
> > > > +        bdi_start_writeback(sb->s_bdi, get_nr_dirty_pages());
> > > 
> > > I'll put on my skis and channel Christoph for a minute.  This isn't
> > > quite the same as the original.  writeback_inodes_sb() writes inodes on a
> > > specific super block, while bdi_start_writeback() writes them for any SB
> > > on that bdi.
> > 
> > Right.
> > 
> > > For btrfs there's only one bdi per SB, but for most everyone else a disk
> > > with a bunch of partitions is going to have multiple filesystems on the
> > > same bdi.
> > > 
> > > 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.

 
> 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).

 
> > 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.

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