On Tue, 26 Jun 2012, Glauber Costa wrote:

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This flag is used to indicate to the callees that this allocation will be
serviced to the kernel. It is not supposed to be passed by the callers
of kmem_cache_alloc, but rather by the cache core itself.

Not sure what "serviced to the kernel" means, does this mean that the
memory will not be accounted for to the root memcg?

In this context, it means that is a kernel allocation, not a userspace one
(but in process context, of course), *and* it is to be accounted a
specific memcg.

Ah, that makes sense.  I think it would help if this was included in the 
changelog as well as a specifying that it is accounted to current's memcg 
at the time of the allocation in a comment in the code.