mm/hugetlb: add cache of descriptors to resv_map for region_add

mm/hugetlb: add cache of descriptors to resv_map for region_add

Orabug: 21652814

hugetlbfs is used today by applications that want a high degree of control
over huge page usage.  Often, large hugetlbfs files are used to map a
large number huge pages into the application processes.  The applications
know when page ranges within these large files will no longer be used, and
ideally would like to release them back to the subpool or global pools for
other uses.  The fallocate() system call provides an interface for
preallocation and hole punching within files.  This patch set adds
fallocate functionality to hugetlbfs.

fallocate hole punch will want to remove a specific range of pages.  When
pages are removed, their associated entries in the region/reserve map will
also be removed.  This will break an assumption in the
region_chg/region_add calling sequence.  If a new region descriptor must
be allocated, it is done as part of the region_chg processing.  In this
way, region_add can not fail because it does not need to attempt an
allocation.

To prepare for fallocate hole punch, create a "cache" of descriptors that
can be used by region_add if necessary.  region_chg will ensure there are
sufficient entries in the cache.  It will be necessary to track the number
of in progress add operations to know a sufficient number of descriptors
reside in the cache.  A new routine region_abort is added to adjust this
in progress count when add operations are aborted.  vma_abort_reservation
is also added for callers creating reservations with
vma_needs_reservation/vma_commit_reservation.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Acked-by: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 27af163113310a86b6d19bb5693c1a08eb89b0f7)
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>