mm, hugetlb: decrement reserve count if VM_NORESERVE alloc page cache

If a vma with VM_NORESERVE allocate a new page for page cache, we should
check whether this area is reserved or not.  If this address is already
reserved by other process(in case of chg == 0), we should decrement
reserve count, because this allocated page will go into page cache and
currently, there is no way to know that this page comes from reserved pool
or not when releasing inode.  This may introduce over-counting problem to
reserved count.  With following example code, you can easily reproduce
this situation.

Assume 2MB, nr_hugepages = 100

        size = 20 * MB;
        flag = MAP_SHARED;
        p = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0);
        if (p == MAP_FAILED) {
                fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
                return -1;
        }

        flag = MAP_SHARED | MAP_NORESERVE;
        q = mmap(NULL, size, PROT_READ|PROT_WRITE, flag, fd, 0);
        if (q == MAP_FAILED) {
                fprintf(stderr, "mmap() failed: %s\n", strerror(errno));
        }
        q[0] = 'c';

After finish the program, run 'cat /proc/meminfo'.  You can see below
result.

HugePages_Free:      100
HugePages_Rsvd:        1

To fix this, we should check our mapping type and tracked region.  If our
mapping is VM_NORESERVE, VM_MAYSHARE and chg is 0, this imply that current
allocated page will go into page cache which is already reserved region
when mapping is created.  In this case, we should decrease reserve count.
As implementing above, this patch solve the problem.

[akpm@linux-foundation.org: fix spelling in comment]
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Hillf Danton <dhillf@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>