gigantic hugepages vs. movable zones

[Michal Hocko]

Hi,
I've just noticed that alloc_gigantic_page ignores movability of the
gigantic page and it uses any existing zone. Considering that
hugepage_migration_supported only supports 2MB and pgd level hugepages
then 1GB pages are not migratable and as such allocating them from a
movable zone will break the basic expectation of this zone. Standard
hugetlb allocations try to avoid that by using htlb_alloc_mask and I
believe we should do the same for gigantic pages as well.

I suspect this behavior is not intentional. What do you think about the
following untested patch?
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