Re: [patch -mm 4/5] mm: test and set zone reclaim lock before starting reclaim

[David Rientjes <rientjes@google.com>]

On Tue, 25 Sep 2007, Balbir Singh wrote:

> > One thing that has been changed in -mm with regard to my last patchset is 
> > that kswapd and try_to_free_pages() are allowed to call shrink_zone() 
> > concurrently.
> > 
> 
> Aah.. interesting. Could you define concurrently more precisely,
> concurrently as in the same zone or for different zones concurrently?
> 

Same zone, thankfully.

Previous to my 9-patch series that serialized the OOM killer, there was an 
atomic_t reclaim_in_progress member of each struct zone that was 
incremented each time shrink_zone() was called and decremented each time 
it exited.  The only place where this was tested was in zone_reclaim() and 
it returned 0 to __alloc_pages() if it was non-zero prior to calling 
__zone_reclaim().

So other callers to shrink_zone(), such as kswapd and try_to_free_pages(), 
were still able to invoke it several times for the same zone but 
zone_reclaim() could not if the zone was being shrunk, regardless of where 
shrink_zone() was called from. 

That's partially still true: kswapd and try_to_free_pages() (and actually 
balance_pgdat()) can still call it several times, but the first call to 
zone_reclaim() will also succeed since we're now indicating zone reclaims 
with a flag instead of an accumulator.

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