Re: [PATCH] [1/4] SLAB: Handle node-not-up case infallback_alloc()

[David Rientjes]

On Sat, 6 Feb 2010, Andi Kleen wrote:

> > That other node must be allowed by current's cpuset, otherwise 
> > kmem_getpages() will fail when get_page_from_freelist() iterates only over 
> > unallowed nodes.
> 
> All theses cases are really only interesting in the memory hotplug path
> itself (afterwards the slab is working anyways and memory is there)
> and if someone sets funny cpusets for those he gets what he deserves ...
> 

If a hot-added node has not been initialized for the cache, your code is 
picking an existing one in zonelist order which may be excluded by 
current's cpuset.  Thus, your code has a very real chance of having 
kmem_getpages() return NULL because get_page_from_freelist() will reject 
non-atomic ALLOC_CPUSET allocations for prohibited nodes.  That isn't a 
scenario that requires a "funny cpuset," it just has to not allow whatever 
initialized node comes first in the zonelist.

My suggested alternative does not pick a single initialized node, rather 
it tries all nodes that actually have a chance of having kmem_getpages() 
succeed which increases the probability that your patch actually has an 
effect for cpuset users.
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