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/**
* @file nodes.c
* The node subsystem. Each client has to initialize their own node system,
* after which they can request nodes of a size specified at init.
*
* Node allocation is implemented through a similar system used by jemalloc
* (https://github.com/jemalloc/jemalloc), but instead of having a number of
* different sized buckets there is only the one size specified by the user.
* This cuts down on complexity and improves performance, at a somewhat major
* flexibility cost. Still, this kernel generally only allocates nodes of the
* same size again and again, and this approach seems sensible.
*
*
* Quick overview of the allocator, all nodes live in memory pages. Each memory
* page has a small header at the front, with some metadata about number of free
* and used node slots. When a memory page is filled, a new one is allocated by
* the physical memory subsystem and the pages are linked together in a common
* list. At the same time, a second linked list is maintained which maintains
* which pages have empty slots. When a node is freed, the page it belonged to
* is added to the free list (if it didn't already exist there) and when a new
* node is requested, the free list is looked through first.
*
* \todo More in-depth documentation about the node algorithm.
*/
#include <apos/mem.h>
#include <apos/pmem.h>
#include <apos/bits.h>
#include <apos/nodes.h>
#include <apos/string.h>
/* the structure of each node_region is approximately
*
* struct node_region | bitmap | array of node_size nodes
*
* where array starts on a multiple of node_size to ensure alignment and
* bitmap is a bitmap of whether node at index is free or used (1 being used, 0
* being free)
*/
#define node_region(r) \
((struct node_region *)((uintptr_t)(r) & ~(BASE_PAGE_SIZE - 1)))
static struct node_region *__create_region()
{
struct node_region *r = (struct node_region *)alloc_page(BASE_PAGE, 0);
memset(r, FREE, BASE_PAGE_SIZE);
return r;
}
void init_nodes(struct node_root *r, size_t node_size)
{
r->head = __create_region();
r->av_head = r->head;
r->node_size = node_size;
r->bitmap = sizeof(struct node_region);
/* ideal values */
size_t max_nodes = BASE_PAGE_SIZE / node_size;
/* make sure not to truncate division */
size_t bitmap_size = (max_nodes / 8) + 1;
uintptr_t first_node = r->bitmap + bitmap_size;
/* actual values */
r->first_node = align_up(first_node, node_size);
r->max_nodes = max_nodes - (r->first_node / node_size);
}
void destroy_nodes(struct node_root *r)
{
struct node_region *nr = r->head;
while (nr) {
struct node_region *d = nr;
nr = nr->prev;
free_page(BASE_PAGE, (pm_t)d);
}
}
static void *__find_free_node(struct node_root *r, struct node_region *nr)
{
uint8_t *bitmap = r->bitmap + (uint8_t *)nr;
for (size_t i = 0; i < r->max_nodes; ++i) {
if (bitmap_is_set(bitmap, i))
continue;
bitmap_set(bitmap, i);
return (i * r->node_size) + (r->first_node + (uint8_t *)nr);
}
return 0;
}
static void __pop_av_head(struct node_root *r)
{
struct node_region *t = r->av_head;
r->av_head = r->av_head->next;
if (r->av_head)
r->av_head->av_prev = 0;
t->av_next = 0;
t->av_prev = 0;
}
void *get_node(struct node_root *r)
{
if (!r)
return 0;
if (!r->av_head) {
r->av_head = __create_region();
r->av_head->prev = r->head;
r->head->next = r->av_head;
r->head = r->av_head;
}
void *p = __find_free_node(r, r->av_head);
if (++r->av_head->used_nodes == r->max_nodes)
__pop_av_head(r);
return p;
}
static void __push_av_head(struct node_root *r, struct node_region *nr)
{
nr->av_prev = 0;
nr->av_next = r->av_head;
if (r->av_head)
r->av_head->av_prev = nr;
r->av_head = nr;
}
static void __free_region(struct node_root *r, struct node_region *nr)
{
struct node_region *av_n = nr->av_next;
struct node_region *av_p = nr->av_prev;
if (av_n)
av_n->av_prev = av_p;
if (av_p)
av_p->av_next = av_n;
if (nr == r->av_head)
__pop_av_head(r);
struct node_region *n = nr->next;
struct node_region *p = nr->prev;
if (n)
n->prev = p;
if (p)
p->next = n;
if (nr == r->head) {
if (r->head->prev) {
r->head->next = 0;
r->head = r->head->prev;
} else
return;
}
free_page(BASE_PAGE, (pm_t)nr);
}
void free_node(struct node_root *r, void *p)
{
struct node_region *nr = node_region(p);
uint8_t *bitmap = r->bitmap + (uint8_t *)nr;
size_t i =
((uintptr_t)p - (r->first_node + (uintptr_t)nr)) / r->node_size;
bitmap_clear(bitmap, i);
if (--nr->used_nodes == 0) {
__free_region(r, nr);
return;
}
else if (!nr->av_next && !nr->av_prev)
__push_av_head(r, nr);
}
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