include/linux/flex_array.h

#ifndef _FLEX_ARRAY_H
#define _FLEX_ARRAY_H

#include <linux/types.h>
#include <asm/page.h>

#define FLEX_ARRAY_PART_SIZE PAGE_SIZE
#define FLEX_ARRAY_BASE_SIZE PAGE_SIZE

struct flex_array_part;

/*
 * This is meant to replace cases where an array-like
 * structure has gotten too big to fit into kmalloc()
 * and the developer is getting tempted to use
 * vmalloc().
 */

struct flex_array {
	union {
		struct {
			int element_size;
			int total_nr_elements;
			struct flex_array_part *parts[];
		};
		/*
		 * This little trick makes sure that
		 * sizeof(flex_array) == PAGE_SIZE
		 */
		char padding[FLEX_ARRAY_BASE_SIZE];
	};
};

/* Number of bytes left in base struct flex_array, excluding metadata */
#define FLEX_ARRAY_BASE_BYTES_LEFT					\
	(FLEX_ARRAY_BASE_SIZE - offsetof(struct flex_array, parts))

/* Number of pointers in base to struct flex_array_part pages */
#define FLEX_ARRAY_NR_BASE_PTRS						\
	(FLEX_ARRAY_BASE_BYTES_LEFT / sizeof(struct flex_array_part *))

/* Number of elements of size that fit in struct flex_array_part */
#define FLEX_ARRAY_ELEMENTS_PER_PART(size)				\
	(FLEX_ARRAY_PART_SIZE / size)

/*
 * Defines a statically allocated flex array and ensures its parameters are
 * valid.
 */
#define DEFINE_FLEX_ARRAY(__arrayname, __element_size, __total)		\
	struct flex_array __arrayname = { { {				\
		.element_size = (__element_size),			\
		.total_nr_elements = (__total),				\
	} } };								\
	static inline void __arrayname##_invalid_parameter(void)	\
	{								\
		BUILD_BUG_ON((__total) > FLEX_ARRAY_NR_BASE_PTRS *	\
			FLEX_ARRAY_ELEMENTS_PER_PART(__element_size));	\
	}

struct flex_array *flex_array_alloc(int element_size, unsigned int total,
		gfp_t flags);
int flex_array_prealloc(struct flex_array *fa, unsigned int start,
		unsigned int end, gfp_t flags);
void flex_array_free(struct flex_array *fa);
void flex_array_free_parts(struct flex_array *fa);
int flex_array_put(struct flex_array *fa, unsigned int element_nr, void *src,
		gfp_t flags);
int flex_array_clear(struct flex_array *fa, unsigned int element_nr);
void *flex_array_get(struct flex_array *fa, unsigned int element_nr);
int flex_array_shrink(struct flex_array *fa);

#endif /* _FLEX_ARRAY_H */