Re: A unresponsive file system can hang all I/O in the system on linux-2.6.23-rc6 (dirty_thresh problem?)

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

On Sat, Sep 29, 2007 at 01:48:01PM +0200, Peter Zijlstra wrote:
> 
> On Sat, 2007-09-29 at 19:04 +0800, Fengguang Wu wrote:
> > On Thu, Sep 27, 2007 at 11:32:36PM -0700, Chakri n wrote:
> > > Hi,
> > > 
> > > In my testing, a unresponsive file system can hang all I/O in the system.
> > > This is not seen in 2.4.
> > > 
> > > I started 20 threads doing I/O on a NFS share. They are just doing 4K
> > > writes in a loop.
> > > 
> > > Now I stop NFS server hosting the NFS share and start a
> > > "dd" process to write a file on local EXT3 file system.
> > > 
> > > # dd if=/dev/zero of=/tmp/x count=1000
> > > 
> > > This process never progresses.
> > 
> > Peter, do you think this patch will help?
> 
> In another sub-thread:
> 
> > It's works on .23-rc8-mm2 with out any problems.
> > 
> > "dd" process does not hang any more.
> > 
> > Thanks for all the help.
> > 
> > Cheers
> > --Chakri
> 
> So the per-bdi dirty patches that are in -mm already fix the problem.

That's good.
But still it could be a good candidate for 2.6.22.x or even 2.6.23.

> > ===
> > writeback: avoid possible balance_dirty_pages() lockup on light-load bdi
> > 
> > On a busy-writing system, a writer could be hold up infinitely on a
> > light-load device. It will be trying to sync more than enough dirty data.
> > 
> > The problem case:
> > 
> > 0. sda/nr_dirty >= dirty_limit;
> >    sdb/nr_dirty == 0
> > 1. dd writes 32 pages on sdb
> > 2. balance_dirty_pages() blocks dd, and tries to write 6MB.
> > 3. it never gets there: there's only 128KB dirty data.
> > 4. dd may be blocked for a loooong time as long as sda is overloaded
> > 
> > Fix it by returning on 'zero dirty inodes' in the current bdi.
> > (In fact there are slight differences between 'dirty inodes' and 'dirty pages'.
> > But there is no available counters for 'dirty pages'.)
> > 
> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
> > ---
> >  mm/page-writeback.c |    3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > --- linux-2.6.22.orig/mm/page-writeback.c
> > +++ linux-2.6.22/mm/page-writeback.c
> > @@ -227,6 +227,9 @@ static void balance_dirty_pages(struct a
> >  		if (nr_reclaimable + global_page_state(NR_WRITEBACK) <=
> >  			dirty_thresh)
> >  				break;
> > +		if (list_empty(&mapping->host->i_sb->s_dirty) &&
> > +		    list_empty(&mapping->host->i_sb->s_io))
> > +			break;
> >  
> >  		if (!dirty_exceeded)
> >  			dirty_exceeded = 1;
> > 
> 
> On the patch itself, not sure if it would have been enough. As soon as
> there is a single dirty inode on the list one would get caught in the
> same problem as before.

That should not be a problem.  Normally the few new dirty inodes will
be all cleaned in one go and there are no more dirty inodes left(at
least for a moment). Hmm, I guess the new 'break' should be moved
immediately after writeback_inodes()...

> That is, if NFS_dirty+NFS_unstable+NFS_writeback > dirty_limit this
> break won't fix it.

In fact this patch exactly targets at this condition.
When NFS* < dirty_limit, Chakri won't see the lockup at all.
The problem was, there are only two 'break's in the loop, and neither
one evaluates to true for his dd command.