Re: [patch for-3.2-rc3] cpusets: stall when updating mems_allowedfor mempolicy or disjoint nodemask

[Miao Xie]

On Tue, 22 Nov 2011 22:25:46 -0800 (pst), David Rientjes wrote:
> On Wed, 23 Nov 2011, Miao Xie wrote:
> 
>> This is a good idea. But I worry that oom will happen easily, because we do
>> direct reclamation and compact by mems_allowed.
>>
> 
> Memory compaction actually iterates through each zone regardless of 
> whether it's allowed or not in the current context.  Recall that the 
> nodemask passed into __alloc_pages_nodemask() is non-NULL only when there 
> is a mempolicy that restricts the allocations by MPOL_BIND.  That nodemask 
> is not protected by get_mems_allowed(), so there's no change in 
> compaction's behavior with my patch.

That nodemask is also protected by get_mems_allowed().

> Direct reclaim does, however, require mems_allowed staying constant 
> without the risk of early oom as you mentioned.  It has its own 
> get_mems_allowed(), though, so it doesn't have the opportunity to change 
> until returning to the page allocator.  It's possible that mems_allowed 
> will be different on the next call to get_pages_from_freelist() but we 
> don't know anything about that context: it's entirely possible that the 
> set of new mems has an abundance of free memory or are completely depleted 
> as well.  So there's no strict need for consistency between the set of 
> allowed nodes during reclaim and the subsequent allocation attempt.  All 
> we care about is that reclaim has a consistent set of allowed nodes to 
> determine whether it's making progress or not.
> 

Agree.
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