Re: [patch 6/4] oom: pass null to kfree if zonelist is not cleared

[David Rientjes <rientjes@google.com>]

On Tue, 18 Sep 2007, Christoph Lameter wrote:

> > Wrong.  Notice what the newly-named try_set_zone_oom() function returns if 
> > the kzalloc() fails; this was a specific design decision.  It returns 1, 
> > so the conditional in __alloc_pages() fails and the OOM killer progresses 
> > as normal.
> 
> So if kzalloc fails then we think that the zone is already running an oom 
> killer while it may only be active on other zones? Doesnt that create more 
> trouble?
> 

If the kzalloc fails, we're in a system-wide OOM state that isn't 
constrained by anything so we allow the OOM killer to be invoked just 
like this patchset was never applied.  We make no inference that it has 
already been invoked, there is nothing to suggest that it has.  All we 
know is that none of the zones in the zonelist from __alloc_pages() are 
currently in the OOM killer.

So we allow the OOM killer to proceed and trust that its heuristics will 
indeed kill a memory-hogging task and free up memory so we can at least 
start kmalloc'ing memory again.  The kernel seems to like that ability.

So the bottomline is that if the kzalloc fails, this entire patchset 
becomes a no-op for that OOM killer invocation; we allow out_of_memory() 
to be called and don't save the zonelist pointer.  I think you'll agree 
that if a kzalloc fails for such a small amount of memory that 
serialization of the OOM killer is the last thing we need to be concerned 
about.

		David

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