mm, hugetlbfs: fix rmapping for anonymous hugepages with page_pgoff()

mm, hugetlbfs: fix rmapping for anonymous hugepages with page_pgoff()

page->index stores pagecache index when the page is mapped into file
mapping region, and the index is in pagecache size unit, so it depends on
the page size.  Some of users of reverse mapping obviously assumes that
page->index is in PAGE_CACHE_SHIFT unit, so they don't work for anonymous
hugepage.

For example, consider that we have 3-hugepage vma and try to mbind the 2nd
hugepage to migrate to another node.  Then the vma is split and
migrate_page() is called for the 2nd hugepage (belonging to the middle
vma.) In migrate operation, rmap_walk_anon() tries to find the relevant
vma to which the target hugepage belongs, but here we miscalculate pgoff.
So anon_vma_interval_tree_foreach() grabs invalid vma, which fires
VM_BUG_ON.

This patch introduces a new API that is usable both for normal page and
hugepage to get PAGE_SIZE offset from page->index.  Users should clearly
distinguish page_index for pagecache index and page_pgoff for page offset.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: <stable@vger.kernel.org>	[3.12+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>