Re: [PATCH 05/10] vmscan: Synchrounous lumpy reclaim use lock_page() instead trylock_page()

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Sep 09, 2010 at 01:13:22PM +0900, KOSAKI Motohiro wrote:
> > On Thu, 9 Sep 2010 12:04:48 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > 
> > > On Mon,  6 Sep 2010 11:47:28 +0100
> > > Mel Gorman <mel@csn.ul.ie> wrote:
> > > 
> > > > From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > > > 
> > > > With synchrounous lumpy reclaim, there is no reason to give up to reclaim
> > > > pages even if page is locked. This patch uses lock_page() instead of
> > > > trylock_page() in this case.
> > > > 
> > > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > > 
> > > Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > 
> > Ah......but can't this change cause dead lock ??
> 
> Yes, this patch is purely crappy. please drop. I guess I was poisoned
> by poisonous mushroom of Mario Bros.
> 

Lets be clear on what the exact dead lock conditions are. The ones I had
thought about when I felt this patch was ok were;

o We are not holding the LRU lock (or any lock, we just called cond_resched())
o We do not have another page locked because we cannot lock multiple pages
o Kswapd will never be in LUMPY_MODE_SYNC so it is not getting blocked
o lock_page() itself is not allocating anything that we could recurse on

One potential dead lock would be if the direct reclaimer held a page
lock and ended up here but is that situation even allowed? I did not
think of an obvious example of when this would happen. Similarly,
deadlock situations with mmap_sem shouldn't happen unless multiple page
locks are being taken.

(prepares to feel foolish)

What did I miss?

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab