1/* SPDX-License-Identifier: GPL-2.0-only */
  2/*
  3 * Copyright 2024 Google LLC
  4 *
  5 * dbitmap - dynamically sized bitmap library.
  6 *
  7 * Used by the binder driver to optimize the allocation of the smallest
  8 * available descriptor ID. Each bit in the bitmap represents the state
  9 * of an ID.
 10 *
 11 * A dbitmap can grow or shrink as needed. This part has been designed
 12 * considering that users might need to briefly release their locks in
 13 * order to allocate memory for the new bitmap. These operations then,
 14 * are verified to determine if the grow or shrink is sill valid.
 15 *
 16 * This library does not provide protection against concurrent access
 17 * by itself. Binder uses the proc->outer_lock for this purpose.
 18 */
 19
 20#ifndef _LINUX_DBITMAP_H
 21#define _LINUX_DBITMAP_H
 22#include <linux/bitmap.h>
 23
 24#define NBITS_MIN	BITS_PER_TYPE(unsigned long)
 25
 26struct dbitmap {
 27	unsigned int nbits;
 28	unsigned long *map;
 29};
 30
 31static inline int dbitmap_enabled(struct dbitmap *dmap)
 32{
 33	return !!dmap->nbits;
 34}
 35
 36static inline void dbitmap_free(struct dbitmap *dmap)
 37{
 38	dmap->nbits = 0;
 39	kfree(dmap->map);
 40}
 41
 42/* Returns the nbits that a dbitmap can shrink to, 0 if not possible. */
 43static inline unsigned int dbitmap_shrink_nbits(struct dbitmap *dmap)
 44{
 45	unsigned int bit;
 46
 47	if (dmap->nbits <= NBITS_MIN)
 48		return 0;
 49
 50	/*
 51	 * Determine if the bitmap can shrink based on the position of
 52	 * its last set bit. If the bit is within the first quarter of
 53	 * the bitmap then shrinking is possible. In this case, the
 54	 * bitmap should shrink to half its current size.
 55	 */
 56	bit = find_last_bit(dmap->map, dmap->nbits);
 57	if (bit < (dmap->nbits >> 2))
 58		return dmap->nbits >> 1;
 59
 60	/* find_last_bit() returns dmap->nbits when no bits are set. */
 61	if (bit == dmap->nbits)
 62		return NBITS_MIN;
 63
 64	return 0;
 65}
 66
 67/* Replace the internal bitmap with a new one of different size */
 68static inline void
 69dbitmap_replace(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
 70{
 71	bitmap_copy(new, dmap->map, min(dmap->nbits, nbits));
 72	kfree(dmap->map);
 73	dmap->map = new;
 74	dmap->nbits = nbits;
 75}
 76
 77static inline void
 78dbitmap_shrink(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
 79{
 80	if (!new)
 81		return;
 82
 83	/*
 84	 * Verify that shrinking to @nbits is still possible. The @new
 85	 * bitmap might have been allocated without locks, so this call
 86	 * could now be outdated. In this case, free @new and move on.
 87	 */
 88	if (!dbitmap_enabled(dmap) || dbitmap_shrink_nbits(dmap) != nbits) {
 89		kfree(new);
 90		return;
 91	}
 92
 93	dbitmap_replace(dmap, new, nbits);
 94}
 95
 96/* Returns the nbits that a dbitmap can grow to. */
 97static inline unsigned int dbitmap_grow_nbits(struct dbitmap *dmap)
 98{
 99	return dmap->nbits << 1;
100}
101
102static inline void
103dbitmap_grow(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
104{
105	/*
106	 * Verify that growing to @nbits is still possible. The @new
107	 * bitmap might have been allocated without locks, so this call
108	 * could now be outdated. In this case, free @new and move on.
109	 */
110	if (!dbitmap_enabled(dmap) || nbits <= dmap->nbits) {
111		kfree(new);
112		return;
113	}
114
115	/*
116	 * Check for ENOMEM after confirming the grow operation is still
117	 * required. This ensures we only disable the dbitmap when it's
118	 * necessary. Once the dbitmap is disabled, binder will fallback
119	 * to slow_desc_lookup_olocked().
120	 */
121	if (!new) {
122		dbitmap_free(dmap);
123		return;
124	}
125
126	dbitmap_replace(dmap, new, nbits);
127}
128
129/*
130 * Finds and sets the next zero bit in the bitmap. Upon success @bit
131 * is populated with the index and 0 is returned. Otherwise, -ENOSPC
132 * is returned to indicate that a dbitmap_grow() is needed.
133 */
134static inline int
135dbitmap_acquire_next_zero_bit(struct dbitmap *dmap, unsigned long offset,
136			      unsigned long *bit)
137{
138	unsigned long n;
139
140	n = find_next_zero_bit(dmap->map, dmap->nbits, offset);
141	if (n == dmap->nbits)
142		return -ENOSPC;
143
144	*bit = n;
145	set_bit(n, dmap->map);
146
147	return 0;
148}
149
150static inline void
151dbitmap_clear_bit(struct dbitmap *dmap, unsigned long bit)
152{
153	clear_bit(bit, dmap->map);
154}
155
156static inline int dbitmap_init(struct dbitmap *dmap)
157{
158	dmap->map = bitmap_zalloc(NBITS_MIN, GFP_KERNEL);
159	if (!dmap->map) {
160		dmap->nbits = 0;
161		return -ENOMEM;
162	}
163
164	dmap->nbits = NBITS_MIN;
165
166	return 0;
167}
168#endif