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 26 struct dbitmap { 27 unsigned int nbits; 28 unsigned long *map; 29 }; 30 31 static inline int dbitmap_enabled(struct dbitmap *dmap) 32 { 33 return !!dmap->nbits; 34 } 35 36 static 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. */ 43 static 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 */ 68 static inline void 69 dbitmap_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 77 static inline void 78 dbitmap_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 The @new 85 * bitmap might have been allocated without locks, so this call 86 * could now be outdated.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. */ 97 static inline unsigned int dbitmap_grow_nbits(struct dbitmap *dmap) 98 { 99 return dmap->nbits << 1; 100 } 101 102 static inline void 103 dbitmap_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 */ 134 static inline int 135 dbitmap_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 } 149 150 static inline void 151 dbitmap_clear_bit(struct dbitmap *dmap, unsigned long bit) 152 { 153 clear_bit(bit, dmap->map); 154 } 155 156 static 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 169