mmap: allow MAP_HUGETLB for hugetlbfs files v2

It is counterintuitive at best that mmap'ing a hugetlbfs file with
MAP_HUGETLB fails, while mmap'ing it without will a) succeed and b)
return huge pages.

v2: use is_file_hugepages(), as suggested by Jianguo
Signed-off-by: NJoern Engel <joern@logfs.org>
Cc: Jianguo Wu <wujianguo@huawei.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1358,13 +1358,14 @@ SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
 
 	if (!(flags & MAP_ANONYMOUS)) {
 		audit_mmap_fd(fd, flags);
-		if (unlikely(flags & MAP_HUGETLB))
-			return -EINVAL;
 		file = fget(fd);
 		if (!file)
 			goto out;
 		if (is_file_hugepages(file))
 			len = ALIGN(len, huge_page_size(hstate_file(file)));
+		retval = -EINVAL;
+		if (unlikely(flags & MAP_HUGETLB && !is_file_hugepages(file)))
+			goto out_fput;
 	} else if (flags & MAP_HUGETLB) {
 		struct user_struct *user = NULL;
 		struct hstate *hs = hstate_sizelog((flags >> MAP_HUGE_SHIFT) &
@@ -1391,6 +1392,7 @@ SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
 	flags &= ~(MAP_EXECUTABLE | MAP_DENYWRITE);
 
 	retval = vm_mmap_pgoff(file, addr, len, prot, flags, pgoff);
+out_fput:
 	if (file)
 		fput(file);
 out: