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/* SPDX-License-Identifier: GPL-2.0-only */
/*
* Copyright 2024 Google LLC
*
* dbitmap - dynamically sized bitmap library.
*
* Used by the binder driver to optimize the allocation of the smallest
* available descriptor ID. Each bit in the bitmap represents the state
* of an ID, with the exception of BIT(0) which is used exclusively to
* reference binder's context manager.
*
* A dbitmap can grow or shrink as needed. This part has been designed
* considering that users might need to briefly release their locks in
* order to allocate memory for the new bitmap. These operations then,
* are verified to determine if the grow or shrink is sill valid.
*
* This library does not provide protection against concurrent access
* by itself. Binder uses the proc->outer_lock for this purpose.
*/
#ifndef _LINUX_DBITMAP_H
#define _LINUX_DBITMAP_H
#include <linux/bitmap.h>
#define NBITS_MIN BITS_PER_TYPE(unsigned long)
struct dbitmap {
unsigned int nbits;
unsigned long *map;
};
static inline int dbitmap_enabled(struct dbitmap *dmap)
{
return !!dmap->nbits;
}
static inline void dbitmap_free(struct dbitmap *dmap)
{
dmap->nbits = 0;
kfree(dmap->map);
}
/* Returns the nbits that a dbitmap can shrink to, 0 if not possible. */
static inline unsigned int dbitmap_shrink_nbits(struct dbitmap *dmap)
{
unsigned int bit;
if (dmap->nbits <= NBITS_MIN)
return 0;
/*
* Determine if the bitmap can shrink based on the position of
* its last set bit. If the bit is within the first quarter of
* the bitmap then shrinking is possible. In this case, the
* bitmap should shrink to half its current size.
*/
bit = find_last_bit(dmap->map, dmap->nbits);
if (bit < (dmap->nbits >> 2))
return dmap->nbits >> 1;
/*
* Note that find_last_bit() returns dmap->nbits when no bits
* are set. While this is technically not possible here since
* BIT(0) is always set, this check is left for extra safety.
*/
if (bit == dmap->nbits)
return NBITS_MIN;
return 0;
}
/* Replace the internal bitmap with a new one of different size */
static inline void
dbitmap_replace(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
{
bitmap_copy(new, dmap->map, min(dmap->nbits, nbits));
kfree(dmap->map);
dmap->map = new;
dmap->nbits = nbits;
}
static inline void
dbitmap_shrink(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
{
if (!new)
return;
/*
* Verify that shrinking to @nbits is still possible. The @new
* bitmap might have been allocated without locks, so this call
* could now be outdated. In this case, free @new and move on.
*/
if (!dbitmap_enabled(dmap) || dbitmap_shrink_nbits(dmap) != nbits) {
kfree(new);
return;
}
dbitmap_replace(dmap, new, nbits);
}
/* Returns the nbits that a dbitmap can grow to. */
static inline unsigned int dbitmap_grow_nbits(struct dbitmap *dmap)
{
return dmap->nbits << 1;
}
static inline void
dbitmap_grow(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
{
/*
* Verify that growing to @nbits is still possible. The @new
* bitmap might have been allocated without locks, so this call
* could now be outdated. In this case, free @new and move on.
*/
if (!dbitmap_enabled(dmap) || nbits <= dmap->nbits) {
kfree(new);
return;
}
/*
* Check for ENOMEM after confirming the grow operation is still
* required. This ensures we only disable the dbitmap when it's
* necessary. Once the dbitmap is disabled, binder will fallback
* to slow_desc_lookup_olocked().
*/
if (!new) {
dbitmap_free(dmap);
return;
}
dbitmap_replace(dmap, new, nbits);
}
/*
* Finds and sets the first zero bit in the bitmap. Upon success @bit
* is populated with the index and 0 is returned. Otherwise, -ENOSPC
* is returned to indicate that a dbitmap_grow() is needed.
*/
static inline int
dbitmap_acquire_first_zero_bit(struct dbitmap *dmap, unsigned long *bit)
{
unsigned long n;
n = find_first_zero_bit(dmap->map, dmap->nbits);
if (n == dmap->nbits)
return -ENOSPC;
*bit = n;
set_bit(n, dmap->map);
return 0;
}
static inline void
dbitmap_clear_bit(struct dbitmap *dmap, unsigned long bit)
{
/* BIT(0) should always set for the context manager */
if (bit)
clear_bit(bit, dmap->map);
}
static inline int dbitmap_init(struct dbitmap *dmap)
{
dmap->map = bitmap_zalloc(NBITS_MIN, GFP_KERNEL);
if (!dmap->map) {
dmap->nbits = 0;
return -ENOMEM;
}
dmap->nbits = NBITS_MIN;
/* BIT(0) is reserved for the context manager */
set_bit(0, dmap->map);
return 0;
}
#endif
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