mm/gup: have internal functions get the mmap_read_lock()

mm/gup: have internal functions get the mmap_read_lock()

Patch series "Simplify the external interface for GUP", v2.

It is quite a maze of EXPORTED symbols leading up to the three actual
worker functions of GUP. Simplify this by reorganizing some of the code so
the EXPORTED symbols directly call the correct internal function with
validated and consistent arguments.

Consolidate all the assertions into one place at the top of the call
chains.

Remove some dead code.

Move more things into the mm/internal.h header

This patch (of 13):

__get_user_pages_locked() and __gup_longterm_locked() both require the
mmap lock to be held.  They have a slightly unusual locked parameter that
is used to allow these functions to unlock and relock the mmap lock and
convey that fact to the caller.

Several places wrap these functions with a simple mmap_read_lock() just so
they can follow the optimized locked protocol.

Consolidate this internally to the functions.  Allow internal callers to
set locked = 0 to cause the functions to acquire and release the lock on
their own.

Reorganize __gup_longterm_locked() to use the autolocking in
__get_user_pages_locked().

Replace all the places obtaining the mmap_read_lock() just to call
__get_user_pages_locked() with the new mechanism.  Replace all the
internal callers of get_user_pages_unlocked() with direct calls to
__gup_longterm_locked() using the new mechanism.

A following patch will add assertions ensuring the external interface
continues to always pass in locked = 1.

Link: https://lkml.kernel.org/r/0-v2-987e91b59705+36b-gup_tidy_jgg@nvidia.com
Link: https://lkml.kernel.org/r/1-v2-987e91b59705+36b-gup_tidy_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>