[PATCH -mm 0/3] fix numa vs kvm scalability issue

[riel@redhat.com]

The NUMA scanning code can end up iterating over many gigabytes
of unpopulated memory, especially in the case of a freshly started
KVM guest with lots of memory.

This results in the mmu notifier code being called even when
there are no mapped pages in a virtual address range. The amount
of time wasted can be enough to trigger soft lockup warnings
with very large (>2TB) KVM guests.

This patch moves the mmu notifier call to the pmd level, which
represents 1GB areas of memory on x86-64. Furthermore, the mmu
notifier code is only called from the address in the PMD where
present mappings are first encountered.

The hugetlbfs code is left alone for now; hugetlb mappings are
not relocatable, and as such are left alone by the NUMA code,
and should never trigger this problem to begin with.

The series also adds a cond_resched to task_numa_work, to
fix another potential latency issue.

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