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

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

[Andrew Morton]

On Thu, 27 Sep 2007 23:32:36 -0700 "Chakri n" <chakriin5@gmail.com> 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.

yup.

> There is plenty of HIGH MEMORY available in the system, but this
> process never progresses.
> 
> ...
> 
> The problem seems to be in balance_dirty_pages, which calculates
> dirty_thresh based on only ZONE_NORMAL. The same scenario works fine
> in 2.4. The dd processes finishes in no time.
> NFS file systems can go offline, due to multiple reasons, a failed
> switch, filer etc, but that should not effect other file systems in
> the machine.
> Can this behavior be fenced?, can the buffer cache be tuned so that
> other processes do not see the effect?

It's unrelated to the actual value of dirty_thresh: if the machine fills up
with dirty (or unstable) NFS pages then eventually new writers will block
until that condition clears.

2.4 doesn't have this problem at low levels of dirty data because 2.4
VFS/MM doesn't account for NFS pages at all.

I'm not sure what we can do about this from a design perspective, really. 
We have data floating about in memory which we're not allowed to discard
and if we allow it to increase without bound it will eventually either
wedge userspace _anyway_ or it will take the machine down, resulting in
data loss.

What it would be nice to do would be to write that data to local disk if
poss, then reclaim it.  Perhaps David Howells' fscache code can do that (or
could be tweaked to do so).

If you really want to fill all memory with pages whic are dirty against a
dead NFS server then you can manually increase
/proc/sys/vm/dirty_background_ratio and dirty_ratio - that should give you
the 2.4 behaviour.


<thinks>

Actually we perhaps could address this at the VFS level in another way. 
Processes which are writing to the dead NFS server will eventually block in
balance_dirty_pages() once they've exceeded the memory limits and will
remain blocked until the server wakes up - that's the behaviour we want.

What we _don't_ want to happen is for other processes which are writing to
other, non-dead devices to get collaterally blocked.  We have patches which
might fix that queued for 2.6.24.  Peter?

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

[Peter Zijlstra]

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On Thu, 2007-09-27 at 23:50 -0700, Andrew Morton wrote:

> What we _don't_ want to happen is for other processes which are writing to
> other, non-dead devices to get collaterally blocked.  We have patches which
> might fix that queued for 2.6.24.  Peter?

Nasty problem, don't do that :-)

But yeah, with per BDI dirty limits we get stuck at whatever ratio that
NFS server/mount (?) has - which could be 100%. Other processes will
then work almost synchronously against their BDIs but it should work.

[ They will lower the NFS-BDI's ratio, but some fancy clipping code will
  limit the other BDIs their dirty limit to not exceed the total limit.
  And with all these NFS pages stuck, that will still be nothing. ]



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

[Chakri n]

Thanks.

The BDI dirty limits sounds like a good idea.

Is there already a patch for this, which I could try?

I believe it works like this,

Each BDI, will have a limit. If the dirty_thresh exceeds the limit,
all the I/O on the block device will be synchronous.

so, if I have sda & a NFS mount, the dirty limit can be different for
each of them.

I can set dirty limit for
 -  sda to be 90% and
 -  NFS mount to be 50%.

So, if the dirty limit is greater than 50%, NFS does synchronously,
but sda can work asynchronously, till dirty limit reaches 90%.

Thanks
--Chakri

On 9/27/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Thu, 2007-09-27 at 23:50 -0700, Andrew Morton wrote:
>
> > What we _don't_ want to happen is for other processes which are writing to
> > other, non-dead devices to get collaterally blocked.  We have patches which
> > might fix that queued for 2.6.24.  Peter?
>
> Nasty problem, don't do that :-)
>
> But yeah, with per BDI dirty limits we get stuck at whatever ratio that
> NFS server/mount (?) has - which could be 100%. Other processes will
> then work almost synchronously against their BDIs but it should work.
>
> [ They will lower the NFS-BDI's ratio, but some fancy clipping code will
>   limit the other BDIs their dirty limit to not exceed the total limit.
>   And with all these NFS pages stuck, that will still be nothing. ]
>
>
>
>

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

[Peter Zijlstra]

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[ please don't top-post! ]

On Fri, 2007-09-28 at 01:27 -0700, Chakri n wrote:

> On 9/27/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > On Thu, 2007-09-27 at 23:50 -0700, Andrew Morton wrote:
> >
> > > What we _don't_ want to happen is for other processes which are writing to
> > > other, non-dead devices to get collaterally blocked.  We have patches which
> > > might fix that queued for 2.6.24.  Peter?
> >
> > Nasty problem, don't do that :-)
> >
> > But yeah, with per BDI dirty limits we get stuck at whatever ratio that
> > NFS server/mount (?) has - which could be 100%. Other processes will
> > then work almost synchronously against their BDIs but it should work.
> >
> > [ They will lower the NFS-BDI's ratio, but some fancy clipping code will
> >   limit the other BDIs their dirty limit to not exceed the total limit.
> >   And with all these NFS pages stuck, that will still be nothing. ]
> >
> Thanks.
> 
> The BDI dirty limits sounds like a good idea.
> 
> Is there already a patch for this, which I could try?

v2.6.23-rc8-mm2

> I believe it works like this,
> 
> Each BDI, will have a limit. If the dirty_thresh exceeds the limit,
> all the I/O on the block device will be synchronous.
> 
> so, if I have sda & a NFS mount, the dirty limit can be different for
> each of them.
> 
> I can set dirty limit for
>  -  sda to be 90% and
>  -  NFS mount to be 50%.
> 
> So, if the dirty limit is greater than 50%, NFS does synchronously,
> but sda can work asynchronously, till dirty limit reaches 90%.

Not quite, the system determines the limit itself in an adaptive
fashion.

  bdi_limit = total_limit * p_bdi

Where p is a faction [0,1], and is determined by the relative writeout
speed of the current BDI vs all other BDIs.

So if you were to have 3 BDIs (sda, sdb and 1 nfs mount), and sda is
idle, and the nfs mount gets twice as much traffic as sdb, the ratios
will look like:

 p_sda: 0
 p_sdb: 1/3
 p_nfs: 2/3

Once the traffic exceeds the write speed of the device we build up a
backlog and stuff gets throttled, so these proportions converge to the
relative write speed of the BDIs when saturated with data.

So what can happen in your case is that the NFS mount is the only one
with traffic is will get a fraction of 1. If it then disconnects like in
your case, it will still have all of the dirty limit pinned for NFS.

However other devices will at that moment try to maintain a limit of 0,
which ends up being similar to a sync mount.

So they'll not get stuck, but they will be slow.

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

[Chakri n]

Thanks for explaining the adaptive logic.

> However other devices will at that moment try to maintain a limit of 0,
> which ends up being similar to a sync mount.
>
> So they'll not get stuck, but they will be slow.
>
>

Sync should be ok, when the situation is bad like this and some one
hijacked all the buffers.

But, I see my simple dd to write 10blocks on local disk never
completes even after 10 minutes.

[root@h46 ~]# dd if=/dev/zero of=/tmp/x count=10

I think the process is completely stuck and is not progressing at all.

Is something going wrong in the calculations where it does not fall
back to sync mode.

Thanks
--Chakri

On 9/28/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> [ please don't top-post! ]
>
> On Fri, 2007-09-28 at 01:27 -0700, Chakri n wrote:
>
> > On 9/27/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > > On Thu, 2007-09-27 at 23:50 -0700, Andrew Morton wrote:
> > >
> > > > What we _don't_ want to happen is for other processes which are writing to
> > > > other, non-dead devices to get collaterally blocked.  We have patches which
> > > > might fix that queued for 2.6.24.  Peter?
> > >
> > > Nasty problem, don't do that :-)
> > >
> > > But yeah, with per BDI dirty limits we get stuck at whatever ratio that
> > > NFS server/mount (?) has - which could be 100%. Other processes will
> > > then work almost synchronously against their BDIs but it should work.
> > >
> > > [ They will lower the NFS-BDI's ratio, but some fancy clipping code will
> > >   limit the other BDIs their dirty limit to not exceed the total limit.
> > >   And with all these NFS pages stuck, that will still be nothing. ]
> > >
> > Thanks.
> >
> > The BDI dirty limits sounds like a good idea.
> >
> > Is there already a patch for this, which I could try?
>
> v2.6.23-rc8-mm2
>
> > I believe it works like this,
> >
> > Each BDI, will have a limit. If the dirty_thresh exceeds the limit,
> > all the I/O on the block device will be synchronous.
> >
> > so, if I have sda & a NFS mount, the dirty limit can be different for
> > each of them.
> >
> > I can set dirty limit for
> >  -  sda to be 90% and
> >  -  NFS mount to be 50%.
> >
> > So, if the dirty limit is greater than 50%, NFS does synchronously,
> > but sda can work asynchronously, till dirty limit reaches 90%.
>
> Not quite, the system determines the limit itself in an adaptive
> fashion.
>
>   bdi_limit = total_limit * p_bdi
>
> Where p is a faction [0,1], and is determined by the relative writeout
> speed of the current BDI vs all other BDIs.
>
> So if you were to have 3 BDIs (sda, sdb and 1 nfs mount), and sda is
> idle, and the nfs mount gets twice as much traffic as sdb, the ratios
> will look like:
>
>  p_sda: 0
>  p_sdb: 1/3
>  p_nfs: 2/3
>
> Once the traffic exceeds the write speed of the device we build up a
> backlog and stuff gets throttled, so these proportions converge to the
> relative write speed of the BDIs when saturated with data.
>
> So what can happen in your case is that the NFS mount is the only one
> with traffic is will get a fraction of 1. If it then disconnects like in
> your case, it will still have all of the dirty limit pinned for NFS.
>
> However other devices will at that moment try to maintain a limit of 0,
> which ends up being similar to a sync mount.
>
> So they'll not get stuck, but they will be slow.
>
>

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

[Peter Zijlstra]

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On Fri, 2007-09-28 at 02:01 -0700, Chakri n wrote:
> Thanks for explaining the adaptive logic.
> 
> > However other devices will at that moment try to maintain a limit of 0,
> > which ends up being similar to a sync mount.
> >
> > So they'll not get stuck, but they will be slow.
> >
> >
> 
> Sync should be ok, when the situation is bad like this and some one
> hijacked all the buffers.
> 
> But, I see my simple dd to write 10blocks on local disk never
> completes even after 10 minutes.
> 
> [root@h46 ~]# dd if=/dev/zero of=/tmp/x count=10
> 
> I think the process is completely stuck and is not progressing at all.
> 
> Is something going wrong in the calculations where it does not fall
> back to sync mode.

What kernel is that?


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

[Chakri n]

It's 2.6.23-rc6.

Thanks
--Chakri

On 9/28/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Fri, 2007-09-28 at 02:01 -0700, Chakri n wrote:
> > Thanks for explaining the adaptive logic.
> >
> > > However other devices will at that moment try to maintain a limit of 0,
> > > which ends up being similar to a sync mount.
> > >
> > > So they'll not get stuck, but they will be slow.
> > >
> > >
> >
> > Sync should be ok, when the situation is bad like this and some one
> > hijacked all the buffers.
> >
> > But, I see my simple dd to write 10blocks on local disk never
> > completes even after 10 minutes.
> >
> > [root@h46 ~]# dd if=/dev/zero of=/tmp/x count=10
> >
> > I think the process is completely stuck and is not progressing at all.
> >
> > Is something going wrong in the calculations where it does not fall
> > back to sync mode.
>
> What kernel is that?
>
>
>

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

[Peter Zijlstra]

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 [ and one copy for the list too ]

On Fri, 2007-09-28 at 02:20 -0700, Chakri n wrote:
> It's 2.6.23-rc6.

Could you try .23-rc8-mm2. It includes the per bdi stuff.

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

[Chakri n]

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


On 9/28/07, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>  [ and one copy for the list too ]
>
> On Fri, 2007-09-28 at 02:20 -0700, Chakri n wrote:
> > It's 2.6.23-rc6.
>
> Could you try .23-rc8-mm2. It includes the per bdi stuff.
>
>

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

[Jonathan Corbet]

Andrew wrote:
> It's unrelated to the actual value of dirty_thresh: if the machine fills up
> with dirty (or unstable) NFS pages then eventually new writers will block
> until that condition clears.
> 
> 2.4 doesn't have this problem at low levels of dirty data because 2.4
> VFS/MM doesn't account for NFS pages at all.

Is it really NFS-related?  I was trying to back up my 2.6.23-rc8 system
to an external USB drive the other day when something flaked and the
drive fell off the bus.  That, too, was sufficient to wedge the entire
system, even though the only thing which needed the dead drive was one
rsync process.  It's kind of a bummer to have to hit the reset button
after the failure of (what should be) a non-critical piece of hardware.

Not that I have a fix to propose...:)

jon

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

[Peter Zijlstra]

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On Fri, 2007-09-28 at 07:28 -0600, Jonathan Corbet wrote:
> Andrew wrote:
> > It's unrelated to the actual value of dirty_thresh: if the machine fills up
> > with dirty (or unstable) NFS pages then eventually new writers will block
> > until that condition clears.
> > 
> > 2.4 doesn't have this problem at low levels of dirty data because 2.4
> > VFS/MM doesn't account for NFS pages at all.
> 
> Is it really NFS-related?  I was trying to back up my 2.6.23-rc8 system
> to an external USB drive the other day when something flaked and the
> drive fell off the bus.  That, too, was sufficient to wedge the entire
> system, even though the only thing which needed the dead drive was one
> rsync process.  It's kind of a bummer to have to hit the reset button
> after the failure of (what should be) a non-critical piece of hardware.
> 
> Not that I have a fix to propose...:)

the per bdi work in -mm should make the system not drop dead.

Still, would a remove,re-insert of the usb media end up with the same
bdi? That is, would it recognise as the same and resume the transfer.

Anyway, it would be grand (and dangerous) if we could provide for a
button that would just kill off all outstanding pages against a dead
device.

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

[Alan Stern]

On Fri, 28 Sep 2007, Peter Zijlstra wrote:

> On Fri, 2007-09-28 at 07:28 -0600, Jonathan Corbet wrote:

> > Is it really NFS-related?  I was trying to back up my 2.6.23-rc8 system
> > to an external USB drive the other day when something flaked and the
> > drive fell off the bus.  That, too, was sufficient to wedge the entire
> > system, even though the only thing which needed the dead drive was one
> > rsync process.  It's kind of a bummer to have to hit the reset button
> > after the failure of (what should be) a non-critical piece of hardware.
> > 
> > Not that I have a fix to propose...:)
> 
> the per bdi work in -mm should make the system not drop dead.
> 
> Still, would a remove,re-insert of the usb media end up with the same
> bdi? That is, would it recognise as the same and resume the transfer.

Removal and replacement of the media might work.  I have never tried 
it.

But Jon described removal of the device, not the media.  Replacing the 
device definitely will not work.

Alan Stern


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

[Daniel Phillips]

On Friday 28 September 2007 06:35, Peter Zijlstra wrote:
> ,,,it would be grand (and dangerous) if we could provide for a
> button that would just kill off all outstanding pages against a dead
> device.

Substitute "resources" for "pages" and you begin to get an idea of how 
tricky that actually is.  That said, this is exactly what we have done 
with ddsnap, for the simple reason that our users, now emboldened by 
being able to stop or terminate the user space part, felt justified in 
expecting that the system continue as if nothing had happened, and 
furthermore, be able to restart ddsnap without a hiccup.  (Otherwise 
known as a sysop's diety-given right to kill.)

So this is what we do in the specific case of ddsnap:

   * When we detect some nasty state change such as our userspace
      control daemon disappearing on us, we go poking around and
      explicitly release every semaphore that the device driver could
      possibly wait on forever (interestingly they are all in our own
      driver except for BKL, which is just an artifact of device mapper
      not having gone over to unlock_ioctl for no good reason that I
      know of).

   * Then at the points were the driver falls through some lock thus
      released, we check our "ready" flag, and if it indicates "busted",
      proceed with wherever cleanup is needed at that point.

Does not sound like an approach one would expect to work reliably, does 
it?  But there just may be some general principle to be ferretted out 
here.  (Anyone who has ideas on how bits of this procedure could be 
abstracted, please do not hesitate to step boldly forth into the 
limelight.)

Incidentally, only a small subset of locks needed special handling as 
above.  Most can be shown to have no way to block forever, short of an 
outright bug.

I shudder to think how much work it would be to bring every driver in 
the kernel up to such a standard, particularly if user space components 
are involved, as with USB.  On the other hand, every driver fixed is 
one less driver that sucks.  The next one to emerge from the pipeline 
will most likely be NBD, which we have been working on in fits and 
starts for a while.  Look for it to morph into "ddbd", with cross-node 
distributed data awareness, in addition to perforning its current job 
without deadlocking.

Regards,

Daniel

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

[Andrew Morton]

On Fri, 28 Sep 2007 07:28:52 -0600 corbet@lwn.net (Jonathan Corbet) wrote:

> Andrew wrote:
> > It's unrelated to the actual value of dirty_thresh: if the machine fills up
> > with dirty (or unstable) NFS pages then eventually new writers will block
> > until that condition clears.
> > 
> > 2.4 doesn't have this problem at low levels of dirty data because 2.4
> > VFS/MM doesn't account for NFS pages at all.
> 
> Is it really NFS-related?  I was trying to back up my 2.6.23-rc8 system
> to an external USB drive the other day when something flaked and the
> drive fell off the bus.  That, too, was sufficient to wedge the entire
> system, even though the only thing which needed the dead drive was one
> rsync process.  It's kind of a bummer to have to hit the reset button
> after the failure of (what should be) a non-critical piece of hardware.
> 
> Not that I have a fix to propose...:)
> 

That's a USB bug, surely.  What should happen is that the kernel attempts
writeback, gets an IO error and then your data gets lost.

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

[Trond Myklebust]

On Thu, 2007-09-27 at 23:50 -0700, Andrew Morton wrote:

> Actually we perhaps could address this at the VFS level in another way. 
> Processes which are writing to the dead NFS server will eventually block in
> balance_dirty_pages() once they've exceeded the memory limits and will
> remain blocked until the server wakes up - that's the behaviour we want.
> 
> What we _don't_ want to happen is for other processes which are writing to
> other, non-dead devices to get collaterally blocked.  We have patches which
> might fix that queued for 2.6.24.  Peter?

Do these patches also cause the memory reclaimers to steer clear of
devices that are congested (and stop waiting on a congested device if
they see that it remains congested for a long period of time)? Most of
the collateral blocking I see tends to happen in memory allocation...

Cheers
  Trond

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

[Andrew Morton]

On Fri, 28 Sep 2007 13:00:53 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> On Thu, 2007-09-27 at 23:50 -0700, Andrew Morton wrote:
> 
> > Actually we perhaps could address this at the VFS level in another way. 
> > Processes which are writing to the dead NFS server will eventually block in
> > balance_dirty_pages() once they've exceeded the memory limits and will
> > remain blocked until the server wakes up - that's the behaviour we want.
> > 
> > What we _don't_ want to happen is for other processes which are writing to
> > other, non-dead devices to get collaterally blocked.  We have patches which
> > might fix that queued for 2.6.24.  Peter?
> 
> Do these patches also cause the memory reclaimers to steer clear of
> devices that are congested (and stop waiting on a congested device if
> they see that it remains congested for a long period of time)? Most of
> the collateral blocking I see tends to happen in memory allocation...
> 

No, they don't attempt to do that, but I suspect they put in place
infrastructure which could be used to improve direct-reclaimer latency.  In
the throttle_vm_writeout() path, at least.

Do you know where the stalls are occurring?  throttle_vm_writeout(), or via
direct calls to congestion_wait() from page_alloc.c and vmscan.c?  (running
sysrq-w five or ten times will probably be enough to determine this)


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

[Peter Zijlstra]

On Fri, 2007-09-28 at 11:49 -0700, Andrew Morton wrote:

> Do you know where the stalls are occurring?  throttle_vm_writeout(), or via
> direct calls to congestion_wait() from page_alloc.c and vmscan.c?  (running
> sysrq-w five or ten times will probably be enough to determine this)

would it make sense to instrument congestion_wait() callsites with
vmstats?

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

[Andrew Morton]

On Fri, 28 Sep 2007 20:48:59 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> 
> On Fri, 2007-09-28 at 11:49 -0700, Andrew Morton wrote:
> 
> > Do you know where the stalls are occurring?  throttle_vm_writeout(), or via
> > direct calls to congestion_wait() from page_alloc.c and vmscan.c?  (running
> > sysrq-w five or ten times will probably be enough to determine this)
> 
> would it make sense to instrument congestion_wait() callsites with
> vmstats?

Better than nothing, but it isn't a great fit: we'd need one vmstat counter
per congestion_wait() callsite, and it's all rather specific to the
kernel-of-the-day.

taskstats delay accounting isn't useful either - it will aggregate all the
schedule() callsites.

profile=sleep is just about ideal for this, isn't it?  I suspect that most
people don't know it's there, or forgot about it.

It could be that profile=sleep just tells us "you're spending a lot of time
in io_schedule()" or congestion_wait(), so perhaps we need to teach it to
go for walk up the stack somehow.

But lockdep knows how to do that already so perhaps we (ie: you ;)) can
bolt sleep instrumentation onto lockdep as we (ie you ;)) did with the
lockstat stuff?

(Searches for the lockstat documentation)

Did we forget to do that?

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

[Trond Myklebust]

[-- Attachment #1: Type: text/plain, Size: 1007 bytes --]

On Fri, 2007-09-28 at 11:49 -0700, Andrew Morton wrote:
> On Fri, 28 Sep 2007 13:00:53 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > Do these patches also cause the memory reclaimers to steer clear of
> > devices that are congested (and stop waiting on a congested device if
> > they see that it remains congested for a long period of time)? Most of
> > the collateral blocking I see tends to happen in memory allocation...
> > 
> 
> No, they don't attempt to do that, but I suspect they put in place
> infrastructure which could be used to improve direct-reclaimer latency.  In
> the throttle_vm_writeout() path, at least.
> 
> Do you know where the stalls are occurring?  throttle_vm_writeout(), or via
> direct calls to congestion_wait() from page_alloc.c and vmscan.c?  (running
> sysrq-w five or ten times will probably be enough to determine this)

Looking back, they were getting caught up in
balance_dirty_pages_ratelimited() and friends. See the attached
example...

Cheers
  Trond

[-- Attachment #2: Attached message - [NFS] NFS on loopback locks up entire system(2.6.23-rc6)? --]
[-- Type: message/rfc822, Size: 9371 bytes --]

From: "Chakri n" <chakriin5@gmail.com>
To: nfs@lists.sourceforge.net, Trond.Myklebust@netapp.com, linux-kernel@vger.kernel.org
Subject: [NFS] NFS on loopback locks up entire system(2.6.23-rc6)?
Date: Thu, 20 Sep 2007 17:22:26 -0700
Message-ID: <92cbf19b0709201722k6265e647x31b7d25bc54b63a0@mail.gmail.com>

Hi,

I am testing NFS on loopback locks up entire system with 2.6.23-rc6 kernel.

I have mounted a local ext3 partition using loopback NFS (version 3)
and started my test program. The test program forks 20 threads
allocates 10MB for each thread, writes & reads a file on the loopback
NFS mount. After running for about 5 min, I cannot even login to the
machine. Commands like ps etc, hang in a live session.

The machine is a DELL 1950 with 4Gig of RAM, so there is plenty of RAM
& CPU to play around and no other io/heavy processes are running on
the system.

vmstat output shows no buffers are actually getting transferred in or
out and iowait is 100%.

[root@h46 ~]# vmstat 1
procs -----------memory---------- ---swap-- -----io---- --system--
-----cpu------
 r  b    swpd       free       buff       cache   si   so   bi   bo
in     cs us sy id wa st
 0 24    116 110080  11132 3045664    0    0     0     0   28  345  0
1  0 99  0
 0 24    116 110080  11132 3045664    0    0     0     0    5  329  0
0  0 100  0
 0 24    116 110080  11132 3045664    0    0     0     0   26  336  0
0  0 100  0
 0 24    116 110080  11132 3045664    0    0     0     0    8  335  0
0  0 100  0
 0 24    116 110080  11132 3045664    0    0     0     0   26  352  0
0  0 100  0
 0 24    116 110080  11132 3045664    0    0     0     0    8  351  0
0  0 100  0
 0 24    116 110080  11132 3045664    0    0     0     0   23  358  0
1  0 99  0
 0 24    116 110080  11132 3045664    0    0     0     0   10  350  0
0  0 100  0
 0 24    116 110080  11132 3045664    0    0     0     0   26  363  0
0  0 100  0
 0 24    116 110080  11132 3045664    0    0     0     0    8  346  0
1  0 99  0
 0 24    116 110080  11132 3045664    0    0     0     0   26  360  0
0  0 100  0
 0 24    116 110080  11140 3045656    0    0     8     0   11  345  0
0  0 100  0
 0 24    116 110080  11140 3045664    0    0     0     0   27  355  0
0  2 97  0
 0 24    116 110080  11140 3045664    0    0     0     0    9  330  0
0  0 100  0
 0 24    116 110080  11140 3045664    0    0     0     0   26  358  0
0  0 100  0


The following is the backtrace of
1. one of the threads of my test program
2. nfsd daemon and
3. a generic command like pstree, after the machine hangs:
-------------------------------------------------------------
crash> bt 3252
PID: 3252   TASK: f6f3c610  CPU: 0   COMMAND: "test"
 #0 [f6bdcc10] schedule at c0624a34
 #1 [f6bdcc84] schedule_timeout at c06250ee
 #2 [f6bdccc8] io_schedule_timeout at c0624c15
 #3 [f6bdccdc] congestion_wait at c045eb7d
 #4 [f6bdcd00] balance_dirty_pages_ratelimited_nr at c045ab91
 #5 [f6bdcd54] generic_file_buffered_write at c0457148
 #6 [f6bdcde8] __generic_file_aio_write_nolock at c04576e5
 #7 [f6bdce40] try_to_wake_up at c042342b
 #8 [f6bdce5c] generic_file_aio_write at c0457799
 #9 [f6bdce8c] nfs_file_write at f8c25cee
#10 [f6bdced0] do_sync_write at c0472e27
#11 [f6bdcf7c] vfs_write at c0473689
#12 [f6bdcf98] sys_write at c0473c95
#13 [f6bdcfb4] sysenter_entry at c0404ddf
    EAX: 00000004  EBX: 00000013  ECX: a4966008  EDX: 00980000
    DS:  007b      ESI: 00980000  ES:  007b      EDI: a4966008
    SS:  007b      ESP: a5ae6ec0  EBP: a5ae6ef0
    CS:  0073      EIP: b7eed410  ERR: 00000004  EFLAGS: 00000246
crash> bt 3188
PID: 3188   TASK: f74c4000  CPU: 1   COMMAND: "nfsd"
 #0 [f6836c7c] schedule at c0624a34
 #1 [f6836cf0] __mutex_lock_slowpath at c062543d
 #2 [f6836d0c] mutex_lock at c0625326
 #3 [f6836d18] generic_file_aio_write at c0457784
 #4 [f6836d48] ext3_file_write at f8888fd7
 #5 [f6836d64] do_sync_readv_writev at c0472d1f
 #6 [f6836e08] do_readv_writev at c0473486
 #7 [f6836e6c] vfs_writev at c047358e
 #8 [f6836e7c] nfsd_vfs_write at f8e7f8d7
 #9 [f6836ee0] nfsd_write at f8e80139
#10 [f6836f10] nfsd3_proc_write at f8e86afd
#11 [f6836f44] nfsd_dispatch at f8e7c20c
#12 [f6836f6c] svc_process at f89c18e0
#13 [f6836fbc] nfsd at f8e7c794
#14 [f6836fe4] kernel_thread_helper at c0405a35
crash> ps|grep ps
    234      2   3  cb194000  IN   0.0       0      0  [khpsbpkt]
    520      2   0  f7e18c20  IN   0.0       0      0  [kpsmoused]
   2859      1   2  f7f3cc20  IN   0.1    9600   2040  cupsd
   3340   3310   0  f4a0f840  UN   0.0    4360    816  pstree
   3343   3284   2  f4a0f230  UN   0.0    4212    944  ps
crash> bt 3340
PID: 3340   TASK: f4a0f840  CPU: 0   COMMAND: "pstree"
 #0 [e856be30] schedule at c0624a34
 #1 [e856bea4] rwsem_down_failed_common at c04df6c0
 #2 [e856bec4] rwsem_down_read_failed at c0625c2a
 #3 [e856bedc] call_rwsem_down_read_failed at c0625c96
 #4 [e856bee8] down_read at c043c21a
 #5 [e856bef0] access_process_vm at c0462039
 #6 [e856bf38] proc_pid_cmdline at c04a1bbb
 #7 [e856bf58] proc_info_read at c04a2f41
 #8 [e856bf7c] vfs_read at c04737db
 #9 [e856bf98] sys_read at c0473c2e
#10 [e856bfb4] sysenter_entry at c0404ddf
    EAX: 00000003  EBX: 00000005  ECX: 0804dc58  EDX: 00000062
    DS:  007b      ESI: 00000cba  ES:  007b      EDI: 0804e0e0
    SS:  007b      ESP: bfa3afe8  EBP: bfa3d4f8
    CS:  0073      EIP: b7f64410  ERR: 00000003  EFLAGS: 00000246
----------------------------------------------------------

Any ideas what could potentially trigger this?

Please let me know if you would like to get any other specific details.

Thanks
--Chakri

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

[Andrew Morton]

On Fri, 28 Sep 2007 15:16:11 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> On Fri, 2007-09-28 at 11:49 -0700, Andrew Morton wrote:
> > On Fri, 28 Sep 2007 13:00:53 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > > Do these patches also cause the memory reclaimers to steer clear of
> > > devices that are congested (and stop waiting on a congested device if
> > > they see that it remains congested for a long period of time)? Most of
> > > the collateral blocking I see tends to happen in memory allocation...
> > > 
> > 
> > No, they don't attempt to do that, but I suspect they put in place
> > infrastructure which could be used to improve direct-reclaimer latency.  In
> > the throttle_vm_writeout() path, at least.
> > 
> > Do you know where the stalls are occurring?  throttle_vm_writeout(), or via
> > direct calls to congestion_wait() from page_alloc.c and vmscan.c?  (running
> > sysrq-w five or ten times will probably be enough to determine this)
> 
> Looking back, they were getting caught up in
> balance_dirty_pages_ratelimited() and friends. See the attached
> example...

that one is nfs-on-loopback, which is a special case, isn't it?

NFS on loopback used to hang, but then we fixed it.  It looks like we
broke it again sometime in the intervening four years or so.

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

[Trond Myklebust]

On Fri, 2007-09-28 at 12:26 -0700, Andrew Morton wrote:
> On Fri, 28 Sep 2007 15:16:11 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > Looking back, they were getting caught up in
> > balance_dirty_pages_ratelimited() and friends. See the attached
> > example...
> 
> that one is nfs-on-loopback, which is a special case, isn't it?

I'm not sure that the hang that is illustrated here is so special. It is
an example of a bog-standard ext3 write, that ends up calling the NFS
client, which is hanging. The fact that it happens to be hanging on the
nfsd process is more or less irrelevant here: the same thing could
happen to any other process in the case where we have an NFS server that
is down.

> NFS on loopback used to hang, but then we fixed it.  It looks like we
> broke it again sometime in the intervening four years or so.

It has been quirky all through the 2.6.x series because of this issue.

Cheers
  Trond


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

[Andrew Morton]

On Fri, 28 Sep 2007 15:52:28 -0400
Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> On Fri, 2007-09-28 at 12:26 -0700, Andrew Morton wrote:
> > On Fri, 28 Sep 2007 15:16:11 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > > Looking back, they were getting caught up in
> > > balance_dirty_pages_ratelimited() and friends. See the attached
> > > example...
> > 
> > that one is nfs-on-loopback, which is a special case, isn't it?
> 
> I'm not sure that the hang that is illustrated here is so special. It is
> an example of a bog-standard ext3 write, that ends up calling the NFS
> client, which is hanging. The fact that it happens to be hanging on the
> nfsd process is more or less irrelevant here: the same thing could
> happen to any other process in the case where we have an NFS server that
> is down.

hm, so ext3 got stuck in nfs via __alloc_pages direct reclaim?

We should be able to fix that by marking the backing device as
write-congested.  That'll have small race windows, but it should be a 99.9%
fix?


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

[Trond Myklebust]

On Fri, 2007-09-28 at 13:10 -0700, Andrew Morton wrote:
> On Fri, 28 Sep 2007 15:52:28 -0400
> Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> 
> > On Fri, 2007-09-28 at 12:26 -0700, Andrew Morton wrote:
> > > On Fri, 28 Sep 2007 15:16:11 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > > > Looking back, they were getting caught up in
> > > > balance_dirty_pages_ratelimited() and friends. See the attached
> > > > example...
> > > 
> > > that one is nfs-on-loopback, which is a special case, isn't it?
> > 
> > I'm not sure that the hang that is illustrated here is so special. It is
> > an example of a bog-standard ext3 write, that ends up calling the NFS
> > client, which is hanging. The fact that it happens to be hanging on the
> > nfsd process is more or less irrelevant here: the same thing could
> > happen to any other process in the case where we have an NFS server that
> > is down.
> 
> hm, so ext3 got stuck in nfs via __alloc_pages direct reclaim?
> 
> We should be able to fix that by marking the backing device as
> write-congested.  That'll have small race windows, but it should be a 99.9%
> fix?

No. The problem would rather appear to be that we're doing
per-backing_dev writeback (if I read sync_sb_inodes() correctly), but
we're measuring variables which are global to the VM. The backing device
that we are selecting may not be writing out any dirty pages, in which
case, we're just spinning in balance_dirty_pages_ratelimited().

Should we therefore perhaps be looking at adding per-backing_dev stats
too?

Trond


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

[Andrew Morton]

On Fri, 28 Sep 2007 16:32:18 -0400
Trond Myklebust <trond.myklebust@fys.uio.no> wrote:

> On Fri, 2007-09-28 at 13:10 -0700, Andrew Morton wrote:
> > On Fri, 28 Sep 2007 15:52:28 -0400
> > Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > 
> > > On Fri, 2007-09-28 at 12:26 -0700, Andrew Morton wrote:
> > > > On Fri, 28 Sep 2007 15:16:11 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > > > > Looking back, they were getting caught up in
> > > > > balance_dirty_pages_ratelimited() and friends. See the attached
> > > > > example...
> > > > 
> > > > that one is nfs-on-loopback, which is a special case, isn't it?
> > > 
> > > I'm not sure that the hang that is illustrated here is so special. It is
> > > an example of a bog-standard ext3 write, that ends up calling the NFS
> > > client, which is hanging. The fact that it happens to be hanging on the
> > > nfsd process is more or less irrelevant here: the same thing could
> > > happen to any other process in the case where we have an NFS server that
> > > is down.
> > 
> > hm, so ext3 got stuck in nfs via __alloc_pages direct reclaim?
> > 
> > We should be able to fix that by marking the backing device as
> > write-congested.  That'll have small race windows, but it should be a 99.9%
> > fix?
> 
> No. The problem would rather appear to be that we're doing
> per-backing_dev writeback (if I read sync_sb_inodes() correctly), but
> we're measuring variables which are global to the VM. The backing device
> that we are selecting may not be writing out any dirty pages, in which
> case, we're just spinning in balance_dirty_pages_ratelimited().

OK, so it's unrelated to page reclaim.

> Should we therefore perhaps be looking at adding per-backing_dev stats
> too?

That's what mm-per-device-dirty-threshold.patch and friends are doing. 
Whether it works adequately is not really known at this time. 
Unfortunately kernel developers don't test -mm much.  

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

[Chakri n]

Here is a the snapshot of vmstats when the problem happened. I believe
this could help a little.

crash> kmem -V
       NR_FREE_PAGES: 680853
         NR_INACTIVE: 95380
           NR_ACTIVE: 26891
       NR_ANON_PAGES: 2507
      NR_FILE_MAPPED: 1832
       NR_FILE_PAGES: 119779
       NR_FILE_DIRTY: 0
        NR_WRITEBACK: 18272
 NR_SLAB_RECLAIMABLE: 1305
NR_SLAB_UNRECLAIMABLE: 2085
        NR_PAGETABLE: 123
     NR_UNSTABLE_NFS: 0
           NR_BOUNCE: 0
     NR_VMSCAN_WRITE: 0

In my testing, I always saw the processes are waiting in
balance_dirty_pages_ratelimited(), never in throttle_vm_writeout()
path.

But this could be because I have about 4Gig of memory in the system
and plenty of mem is still available around.

I will rerun the test limiting memory to 1024MB and lets see if it
takes in any different path.

Thanks
--Chakri


On 9/28/07, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Fri, 28 Sep 2007 16:32:18 -0400
> Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
>
> > On Fri, 2007-09-28 at 13:10 -0700, Andrew Morton wrote:
> > > On Fri, 28 Sep 2007 15:52:28 -0400
> > > Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > >
> > > > On Fri, 2007-09-28 at 12:26 -0700, Andrew Morton wrote:
> > > > > On Fri, 28 Sep 2007 15:16:11 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > > > > > Looking back, they were getting caught up in
> > > > > > balance_dirty_pages_ratelimited() and friends. See the attached
> > > > > > example...
> > > > >
> > > > > that one is nfs-on-loopback, which is a special case, isn't it?
> > > >
> > > > I'm not sure that the hang that is illustrated here is so special. It is
> > > > an example of a bog-standard ext3 write, that ends up calling the NFS
> > > > client, which is hanging. The fact that it happens to be hanging on the
> > > > nfsd process is more or less irrelevant here: the same thing could
> > > > happen to any other process in the case where we have an NFS server that
> > > > is down.
> > >
> > > hm, so ext3 got stuck in nfs via __alloc_pages direct reclaim?
> > >
> > > We should be able to fix that by marking the backing device as
> > > write-congested.  That'll have small race windows, but it should be a 99.9%
> > > fix?
> >
> > No. The problem would rather appear to be that we're doing
> > per-backing_dev writeback (if I read sync_sb_inodes() correctly), but
> > we're measuring variables which are global to the VM. The backing device
> > that we are selecting may not be writing out any dirty pages, in which
> > case, we're just spinning in balance_dirty_pages_ratelimited().
>
> OK, so it's unrelated to page reclaim.
>
> > Should we therefore perhaps be looking at adding per-backing_dev stats
> > too?
>
> That's what mm-per-device-dirty-threshold.patch and friends are doing.
> Whether it works adequately is not really known at this time.
> Unfortunately kernel developers don't test -mm much.
>

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

[Chakri n]

No change in behavior even in case of low memory systems. I confirmed
it running on 1Gig machine.

Thanks
--Chakri

On 9/28/07, Chakri n <chakriin5@gmail.com> wrote:
> Here is a the snapshot of vmstats when the problem happened. I believe
> this could help a little.
>
> crash> kmem -V
>        NR_FREE_PAGES: 680853
>          NR_INACTIVE: 95380
>            NR_ACTIVE: 26891
>        NR_ANON_PAGES: 2507
>       NR_FILE_MAPPED: 1832
>        NR_FILE_PAGES: 119779
>        NR_FILE_DIRTY: 0
>         NR_WRITEBACK: 18272
>  NR_SLAB_RECLAIMABLE: 1305
> NR_SLAB_UNRECLAIMABLE: 2085
>         NR_PAGETABLE: 123
>      NR_UNSTABLE_NFS: 0
>            NR_BOUNCE: 0
>      NR_VMSCAN_WRITE: 0
>
> In my testing, I always saw the processes are waiting in
> balance_dirty_pages_ratelimited(), never in throttle_vm_writeout()
> path.
>
> But this could be because I have about 4Gig of memory in the system
> and plenty of mem is still available around.
>
> I will rerun the test limiting memory to 1024MB and lets see if it
> takes in any different path.
>
> Thanks
> --Chakri
>
>
> On 9/28/07, Andrew Morton <akpm@linux-foundation.org> wrote:
> > On Fri, 28 Sep 2007 16:32:18 -0400
> > Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> >
> > > On Fri, 2007-09-28 at 13:10 -0700, Andrew Morton wrote:
> > > > On Fri, 28 Sep 2007 15:52:28 -0400
> > > > Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > > >
> > > > > On Fri, 2007-09-28 at 12:26 -0700, Andrew Morton wrote:
> > > > > > On Fri, 28 Sep 2007 15:16:11 -0400 Trond Myklebust <trond.myklebust@fys.uio.no> wrote:
> > > > > > > Looking back, they were getting caught up in
> > > > > > > balance_dirty_pages_ratelimited() and friends. See the attached
> > > > > > > example...
> > > > > >
> > > > > > that one is nfs-on-loopback, which is a special case, isn't it?
> > > > >
> > > > > I'm not sure that the hang that is illustrated here is so special. It is
> > > > > an example of a bog-standard ext3 write, that ends up calling the NFS
> > > > > client, which is hanging. The fact that it happens to be hanging on the
> > > > > nfsd process is more or less irrelevant here: the same thing could
> > > > > happen to any other process in the case where we have an NFS server that
> > > > > is down.
> > > >
> > > > hm, so ext3 got stuck in nfs via __alloc_pages direct reclaim?
> > > >
> > > > We should be able to fix that by marking the backing device as
> > > > write-congested.  That'll have small race windows, but it should be a 99.9%
> > > > fix?
> > >
> > > No. The problem would rather appear to be that we're doing
> > > per-backing_dev writeback (if I read sync_sb_inodes() correctly), but
> > > we're measuring variables which are global to the VM. The backing device
> > > that we are selecting may not be writing out any dirty pages, in which
> > > case, we're just spinning in balance_dirty_pages_ratelimited().
> >
> > OK, so it's unrelated to page reclaim.
> >
> > > Should we therefore perhaps be looking at adding per-backing_dev stats
> > > too?
> >
> > That's what mm-per-device-dirty-threshold.patch and friends are doing.
> > Whether it works adequately is not really known at this time.
> > Unfortunately kernel developers don't test -mm much.
> >
>

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

[Daniel Phillips]

On Friday 28 September 2007 12:52, Trond Myklebust wrote:
> I'm not sure that the hang that is illustrated here is so special. It
> is an example of a bog-standard ext3 write, that ends up calling the
> NFS client, which is hanging. The fact that it happens to be hanging
> on the nfsd process is more or less irrelevant here: the same thing
> could happen to any other process in the case where we have an NFS
> server that is down.

Hi Trond,

Could you clarify what you meant by "calling the NFS client"?  I don't 
see any direct call in the posted backtrace.

Regards,

Daniel

KDB?

[Daniel Phillips]

On Friday 28 September 2007 12:16, Trond Myklebust wrote:
> crash> bt 3188

crash> ps|grep ps

Hey, that looks just like kdb!  But I heard that kgdb is better in every 
way than kdb.

Innocently yours,

Daniel

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

[Daniel Phillips]

On Thursday 27 September 2007 23:50, Andrew Morton wrote:
> Actually we perhaps could address this at the VFS level in another
> way. Processes which are writing to the dead NFS server will
> eventually block in balance_dirty_pages() once they've exceeded the
> memory limits and will remain blocked until the server wakes up -
> that's the behaviour we want.

It is not necessary to restrict total dirty pages at all.  Instead it is 
necessary to restrict total writeout in flight.  This is evident from 
the fact that making progress is the one and only reason our kernel 
exists, and writeout is how we make progress clearing memory.  In other 
words, if we guarantee the progress of writeout, we will live happily 
ever after and not have to sell the farm.

The current situation has an eerily similar feeling to the VM 
instability in early 2.4, which was never solved until we convinced 
ourselves that the only way to deal with Moore's law as applied to 
number of memory pages was to implement positive control of swapout in 
the form of reverse mapping[1].  This time round, we need to add 
positive control of writeout in the form of rate limiting.

I _think_ Peter is with me on this, and not only that, but between the 
too of us we already have patches for most of the subsystems that need 
it, and we have both been busy testing (different subsets of) these 
patches to destruction for the better part of a year.

Anyway, to fix the immediate bug before the one true dirty_limit removal 
patch lands (promise) I think you are on the right track by noticing 
that balance_dirty_pages has to become aware of how congested the 
involved block device is, since blocking a writeout process on an 
underused block device is clearly a bad idea.  Note how much this idea 
looks like rate limiting.

[1] We lost the scent for a number of reasons, not least because the 
experimental implementation of reverse mapping at the time was buggy 
for reasons entirely unrelated to the reverse mapping itself.

Regards,

Daniel

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