HugeTLB pages cannot be split, so we use the compound_mapcount to track
rmaps.

Currently page_mapped() will check the compound_mapcount, but will also
go through the constituent pages of a THP compound page and query the
individual _mapcount's too.

Unfortunately, page_mapped() does not distinguish between HugeTLB and
THP compound pages and assumes that a compound page always needs to have
HPAGE_PMD_NR pages querying.

For most cases when dealing with HugeTLB this is just inefficient, but
for scenarios where the HugeTLB page size is less than the pmd block
size (e.g.  when using contiguous bit on ARM) this can lead to crashes.

This patch adjusts the page_mapped function such that we skip the
unnecessary THP reference checks for HugeTLB pages.

Fixes: e1534ae9

 ("mm: differentiate page_mapped() from page_mapcount() for compound pages")
Signed-off-by: Steve Capper <steve.capper@arm.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>