fs: Add FOP_HUGE_PAGES

Instead of checking for specific file_operations, add a bit to
file_operations which denotes a file that only contain hugetlb pages.
This lets us make hugetlbfs_file_operations static, and removes
is_file_shm_hugepages() completely.

Signed-off-by: Matthew Wilcox (Oracle) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/hugetlbfs/inode.c

···
 #include <linux/sched/mm.h>
 
 static const struct address_space_operations hugetlbfs_aops;
-const struct file_operations hugetlbfs_file_operations;
+static const struct file_operations hugetlbfs_file_operations;
 static const struct inode_operations hugetlbfs_dir_inode_operations;
 static const struct inode_operations hugetlbfs_inode_operations;
 
···
 	inode_init_once(&ei->vfs_inode);
 }
 
-const struct file_operations hugetlbfs_file_operations = {
+static const struct file_operations hugetlbfs_file_operations = {
 	.read_iter		= hugetlbfs_read_iter,
 	.mmap			= hugetlbfs_file_mmap,
 	.fsync			= noop_fsync,
 	.get_unmapped_area	= hugetlb_get_unmapped_area,
 	.llseek			= default_llseek,
 	.fallocate		= hugetlbfs_fallocate,
+	.fop_flags		= FOP_HUGE_PAGES,
 };
 
 static const struct inode_operations hugetlbfs_dir_inode_operations = {

include/linux/fs.h

···
 #define FOP_MMAP_SYNC		((__force fop_flags_t)(1 << 2))
 /* Supports non-exclusive O_DIRECT writes from multiple threads */
 #define FOP_DIO_PARALLEL_WRITE	((__force fop_flags_t)(1 << 3))
+/* Contains huge pages */
+#define FOP_HUGE_PAGES		((__force fop_flags_t)(1 << 4))
 
 /* Wrap a directory iterator that needs exclusive inode access */
 int wrap_directory_iterator(struct file *, struct dir_context *,

include/linux/hugetlb.h

···
 	return container_of(inode, struct hugetlbfs_inode_info, vfs_inode);
 }
 
-extern const struct file_operations hugetlbfs_file_operations;
 extern const struct vm_operations_struct hugetlb_vm_ops;
 struct file *hugetlb_file_setup(const char *name, size_t size, vm_flags_t acct,
 				int creat_flags, int page_size_log);
 
-static inline bool is_file_hugepages(struct file *file)
+static inline bool is_file_hugepages(const struct file *file)
 {
-	if (file->f_op == &hugetlbfs_file_operations)
-		return true;
-
-	return is_file_shm_hugepages(file);
+	return file->f_op->fop_flags & FOP_HUGE_PAGES;
 }
 
 static inline struct hstate *hstate_inode(struct inode *i)

include/linux/shm.h

···
 
 long do_shmat(int shmid, char __user *shmaddr, int shmflg, unsigned long *addr,
 	      unsigned long shmlba);
-bool is_file_shm_hugepages(struct file *file);
 void exit_shm(struct task_struct *task);
 #define shm_init_task(task) INIT_LIST_HEAD(&(task)->sysvshm.shm_clist)
 #else
···
 			    unsigned long shmlba)
 {
 	return -ENOSYS;
-}
-static inline bool is_file_shm_hugepages(struct file *file)
-{
-	return false;
 }
 static inline void exit_shm(struct task_struct *task)
 {

ipc/shm.c

···
 };
 
 /*
- * shm_file_operations_huge is now identical to shm_file_operations,
- * but we keep it distinct for the sake of is_file_shm_hugepages().
+ * shm_file_operations_huge is now identical to shm_file_operations
+ * except for fop_flags
  */
 static const struct file_operations shm_file_operations_huge = {
 	.mmap		= shm_mmap,
···
 	.get_unmapped_area	= shm_get_unmapped_area,
 	.llseek		= noop_llseek,
 	.fallocate	= shm_fallocate,
+	.fop_flags	= FOP_HUGE_PAGES,
 };
-
-bool is_file_shm_hugepages(struct file *file)
-{
-	return file->f_op == &shm_file_operations_huge;
-}
 
 static const struct vm_operations_struct shm_vm_ops = {
 	.open	= shm_open,	/* callback for a new vm-area open */