linux-ext4 - Re: [patch] fs: fix deadlocks in writeback_if

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Date:	Thu, 25 Nov 2010 14:53:56 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Jan Kara <jack@...e.cz>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	Chris Mason <chris.mason@...cle.com>,
	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>,
	Eric Sandeen <sandeen@...hat.com>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [patch] fs: fix deadlocks in writeback_if_idle

On Wed, Nov 24, 2010 at 02:10:28PM +0100, Jan Kara wrote:
> 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.

Well, for now, the easiest and simplest fix is my patch, I think. The
objection is that we may not write out anything for the specified sb,
but the current implementation provides no such guarantees at all
anyway, so I don't think it's a big issue.

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