Re: Deadlock possibly caused by too_many_isolated.

[Wu Fengguang]

On Wed, Sep 15, 2010 at 10:23:34AM +0800, Neil Brown wrote:
> On Tue, 14 Sep 2010 20:30:18 -0400
> Rik van Riel <riel@xxxxxxxxxx> wrote:
> 
> > On 09/14/2010 07:11 PM, Neil Brown wrote:
> > 
> > > Index: linux-2.6.32-SLE11-SP1/mm/vmscan.c
> > > ===================================================================
> > > --- linux-2.6.32-SLE11-SP1.orig/mm/vmscan.c	2010-09-15 08:37:32.000000000 +1000
> > > +++ linux-2.6.32-SLE11-SP1/mm/vmscan.c	2010-09-15 08:38:57.000000000 +1000
> > > @@ -1106,6 +1106,11 @@ static unsigned long shrink_inactive_lis
> > >   		/* We are about to die and free our memory. Return now. */
> > >   		if (fatal_signal_pending(current))
> > >   			return SWAP_CLUSTER_MAX;
> > > +		if (!(sc->gfp_mask&  __GFP_IO))
> > > +			/* Not allowed to do IO, so mustn't wait
> > > +			 * on processes that might try to
> > > +			 */
> > > +			return SWAP_CLUSTER_MAX;
> > >   	}
> > >
> > >   	/*
> > 
> > Close.  We must also be sure that processes without __GFP_FS
> > set in their gfp_mask do not wait on processes that do have
> > __GFP_FS set.
> > 
> > Considering how many times we've run into a bug like this,
> > I'm kicking myself for not having thought of it :(
> > 
> 
> So maybe this?  I've added the test for __GFP_FS, and moved the test before
> the congestion_wait on the basis that we really want to get back up the stack
> and try the mempool ASAP.

The patch may well fail the !__GFP_IO page allocation and then
quickly exhaust the mempool.

Another approach may to let too_many_isolated() use much higher
thresholds for !__GFP_IO/FS and lower ones for __GFP_IO/FS. ie. to
allow at least nr2 NOIO/FS tasks to be blocked independent of the
IO/FS ones.  Since NOIO vmscans typically completes fast, it will then
very hard to accumulate enough NOIO processes to be actually blocked.


                  IO/FS tasks                NOIO/FS tasks           full
                  block here                 block here              LRU size
|-----------------|--------------------------|-----------------------|
|      nr1        |           nr2            |


Thanks,
Fengguang

> 
> From: NeilBrown <neilb@xxxxxxx>
> 
> mm: Avoid possible deadlock caused by too_many_isolated()
> 
> 
> If too_many_isolated() returns true while performing direct reclaim we can
> end up waiting for other threads to complete their direct reclaim.
> If those threads are allowed to enter the FS or IO to free memory, but
> this thread is not, then it is possible that those threads will be waiting on
> this thread and so we get a circular deadlock.
> 
> So: if too_many_isolated() returns true when the allocation did not permit FS
> or IO, fail shrink_inactive_list rather than blocking.
> 
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> 
> --- linux-2.6.32-SLE11-SP1.orig/mm/vmscan.c	2010-09-15 08:37:32.000000000 +1000
> +++ linux-2.6.32-SLE11-SP1/mm/vmscan.c	2010-09-15 12:17:16.000000000 +1000
> @@ -1101,6 +1101,12 @@ static unsigned long shrink_inactive_lis
>  	int lumpy_reclaim = 0;
>  
>  	while (unlikely(too_many_isolated(zone, file, sc))) {
> +		if ((sc->gfp_mask & GFP_IOFS) != GFP_IOFS)
> +			/* Not allowed to do IO, so mustn't wait
> +			 * on processes that might try to
> +			 */
> +			return SWAP_CLUSTER_MAX;
> +
>  		congestion_wait(BLK_RW_ASYNC, HZ/10);
>  
>  		/* We are about to die and free our memory. Return now. */
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