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// Physical memory allocator, intended to allocate
// memory for user processes. Allocates in 4096-byte "pages".
// Free list is kept sorted and combines adjacent pages into
// long runs, to make it easier to allocate big segments.
// One reason the page size is 4k is that the x86 segment size
// granularity is 4k.
#include "param.h"
#include "types.h"
#include "defs.h"
#include "param.h"
#include "mmu.h"
#include "proc.h"
#include "spinlock.h"
struct spinlock kalloc_lock;
struct run {
struct run *next;
int len; // bytes
};
struct run *freelist;
// Initialize free list of physical pages.
// This code cheats by just considering one megabyte of
// pages after _end. Real systems would determine the
// amount of memory available in the system and use it all.
void
kinit(void)
{
extern int end;
uint mem;
char *start;
initlock(&kalloc_lock, "kalloc");
start = (char*) &end;
start = (char*) (((uint)start + PAGE) & ~(PAGE-1));
mem = 256; // assume 256 pages of RAM
cprintf("mem = %d\n", mem * PAGE);
kfree(start, mem * PAGE);
}
void
kfree(char *cp, int len)
{
struct run **rr;
struct run *p = (struct run*) cp;
struct run *pend = (struct run*) (cp + len);
int i;
if(len % PAGE)
panic("kfree");
// Fill with junk to catch dangling refs.
for(i = 0; i < len; i++)
cp[i] = 1;
acquire(&kalloc_lock);
rr = &freelist;
while(*rr){
struct run *rend = (struct run*) ((char*)(*rr) + (*rr)->len);
if(p >= *rr && p < rend)
panic("freeing free page");
if(pend == *rr){
p->len = len + (*rr)->len;
p->next = (*rr)->next;
*rr = p;
goto out;
}
if(pend < *rr){
p->len = len;
p->next = *rr;
*rr = p;
goto out;
}
if(p == rend){
(*rr)->len += len;
if((*rr)->next && (*rr)->next == pend){
(*rr)->len += (*rr)->next->len;
(*rr)->next = (*rr)->next->next;
}
goto out;
}
rr = &((*rr)->next);
}
p->len = len;
p->next = 0;
*rr = p;
out:
release(&kalloc_lock);
}
// Allocate n bytes of physical memory.
// Returns a kernel-segment pointer.
// Returns 0 if the memory cannot be allocated.
char*
kalloc(int n)
{
struct run **rr;
if(n % PAGE)
panic("kalloc");
acquire(&kalloc_lock);
rr = &freelist;
while(*rr){
struct run *r = *rr;
if(r->len == n){
*rr = r->next;
release(&kalloc_lock);
return (char*) r;
}
if(r->len > n){
char *p = (char*)r + (r->len - n);
r->len -= n;
release(&kalloc_lock);
return p;
}
rr = &(*rr)->next;
}
release(&kalloc_lock);
cprintf("kalloc: out of memory\n");
return 0;
}
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