Re: dirty balancing deadlock

[Miklos Szeredi <miklos@szeredi.hu>]

> > How about this?
> 
> I still don't understand this bug.
> 
> > Solves the FUSE deadlock, but not the throttle_vm_writeout() one.
> > I'll try to tackle that one as well.
> > 
> > If the per-bdi dirty counter goes below 16, balance_dirty_pages()
> > returns.
> > 
> > Does the constant need to tunable?  If it's too large, then the global
> > threshold is more easily exceeded.  If it's too small, then in a tight
> > situation progress will be slower.
> > 
> > Thanks,
> > Miklos
> > 
> > Index: linux/mm/page-writeback.c
> > ===================================================================
> > --- linux.orig/mm/page-writeback.c	2007-02-19 17:32:41.000000000 +0100
> > +++ linux/mm/page-writeback.c	2007-02-19 18:05:28.000000000 +0100
> > @@ -198,6 +198,25 @@ static void balance_dirty_pages(struct a
> >  			dirty_thresh)
> >  				break;
> >  
> > +		/*
> > +		 * Acquit this producer if there's little or nothing
> > +		 * to write back to this particular queue
> > +		 *
> > +		 * Without this check a deadlock is possible in the
> > +		 * following case:
> > +		 *
> > +		 * - filesystem A writes data through filesystem B
> > +		 * - filesystem A has dirty pages over dirty_thresh
> > +		 * - writeback is started, this triggers a write in B
> > +		 * - balance_dirty_pages() is called synchronously
> > +		 * - the write to B blocks
> > +		 * - the writeback completes, but dirty is still over threshold
> > +		 * - the blocking write prevents futher writes from happening
> > +		 */
> > +		if (atomic_long_read(&bdi->nr_dirty) +
> > +		    atomic_long_read(&bdi->nr_writeback) < 16)
> > +			break;
> > +
> 
> The problem seems to that little "- the write to B blocks".
> 
> How come it blocks?  I mean, if we cannot retire writes to that filesystem
> then we're screwed anyway.

Sorry about the sloppy description.  I mean, it's not the lowlevel
write that will block, but rather the VFS one
(generic_file_aio_write).  It will block (or rather loop forever with
0.1 second sleeps) in balance_dirty_pages().  That means, that for
this inode, i_mutex is held and no other writer can continue the work.

Miklos
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