mm: account for MAP_SHARED mappings using VM_MAYSHARE and not VM_SHARED in hugetlbfs

Addresses http://bugzilla.kernel.org/show_bug.cgi?id=13302

hugetlbfs reserves huge pages but does not fault them at mmap() time to
ensure that future faults succeed.  The reservation behaviour differs
depending on whether the mapping was mapped MAP_SHARED or MAP_PRIVATE.
For MAP_SHARED mappings, hugepages are reserved when mmap() is first
called and are tracked based on information associated with the inode.
Other processes mapping MAP_SHARED use the same reservation.  MAP_PRIVATE
track the reservations based on the VMA created as part of the mmap()
operation.  Each process mapping MAP_PRIVATE must make its own
reservation.

hugetlbfs currently checks if a VMA is MAP_SHARED with the VM_SHARED flag
and not VM_MAYSHARE.  For file-backed mappings, such as hugetlbfs,
VM_SHARED is set only if the mapping is MAP_SHARED and the file was opened
read-write.  If a shared memory mapping was mapped shared-read-write for
populating of data and mapped shared-read-only by other processes, then
hugetlbfs would account for the mapping as if it was MAP_PRIVATE.  This
causes processes to fail to map the file MAP_SHARED even though it should
succeed as the reservation is there.

This patch alters mm/hugetlb.c and replaces VM_SHARED with VM_MAYSHARE
when the intent of the code was to check whether the VMA was mapped
MAP_SHARED or MAP_PRIVATE.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: <stable@kernel.org>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: <starlight@binnacle.cx>
Cc: Eric B Munson <ebmunson@us.ibm.com>
Cc: Adam Litke <agl@us.ibm.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>