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 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.
Signed-off-by: Rik van Riel <riel@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Reported-by: Xing Gang <gang.xing@hp.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>