xv6-cs450/kalloc.c

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/*
* physical memory allocator, intended to be used to allocate
* memory for user processes. allocates in 4096-byte "pages".
* free list is 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;
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struct run {
struct run *next;
int len; // bytes
};
struct run *freelist;
/*
* initialize free list of physical pages. this code
* cheats by just considering the one megabyte of pages
* after _end.
*/
void
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kinit(void)
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{
extern int end;
uint mem;
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char *start;
initlock(&kalloc_lock, "kalloc");
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start = (char *) &end;
start = (char *) (((uint)start + PAGE) & ~(PAGE-1));
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mem = 256; // assume 256 pages of RAM
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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;
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if(len % PAGE)
panic("kfree");
// XXX fill with junk to help debug
for(i = 0; i < len; i++)
cp[i] = 1;
acquire(&kalloc_lock);
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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;
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}
if(pend < *rr){
p->len = len;
p->next = *rr;
*rr = p;
goto out;
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}
if(p == rend){
(*rr)->len += len;
if((*rr)->next && (*rr)->next == pend){
(*rr)->len += (*rr)->next->len;
(*rr)->next = (*rr)->next->next;
}
goto out;
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}
rr = &((*rr)->next);
}
p->len = len;
p->next = 0;
*rr = p;
out:
release(&kalloc_lock);
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}
/*
* allocate n bytes of physical memory.
* returns a kernel-segment pointer.
* returns 0 if there's no run that's big enough.
*/
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char*
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kalloc(int n)
{
struct run **rr;
if(n % PAGE)
panic("kalloc");
acquire(&kalloc_lock);
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rr = &freelist;
while(*rr){
struct run *r = *rr;
if(r->len == n){
*rr = r->next;
release(&kalloc_lock);
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return (char *) r;
}
if(r->len > n){
char *p = (char *)r + (r->len - n);
r->len -= n;
release(&kalloc_lock);
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return p;
}
rr = &(*rr)->next;
}
release(&kalloc_lock);
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cprintf("kalloc: out of memory\n");
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return 0;
}