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// Physical memory allocator, for user processes,
// kernel stacks, page-table pages,
// and pipe buffers. Allocates whole 4096-byte pages.
#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "riscv.h"
#include "defs.h"
void freerange(void *pa_start, void *pa_end);
extern char end[]; // first address after kernel.
// defined by kernel.ld.
struct run {
struct run *next;
};
struct {
struct spinlock lock;
struct run *freelist;
} kmem;
int phypg_refcnt[PHYSTOP/PGSIZE];
// Increase the refcnt
int
refcnt_inc(uint64 pa)
{
acquire(&kmem.lock);
int *prefcnt = &phypg_refcnt[pa/PGSIZE];
if(pa > PHYSTOP || *prefcnt < 1)
panic("increase refcnt");
(*prefcnt)++;
release(&kmem.lock);
return *prefcnt;
}
// Decrease the refcnt
int
refcnt_dec(uint64 pa)
{
acquire(&kmem.lock);
int *prefcnt = &phypg_refcnt[pa/PGSIZE];
if(pa > PHYSTOP || *prefcnt < 1)
panic("decrease refcnt");
(*prefcnt)--;
release(&kmem.lock);
return *prefcnt;
}
void
kinit()
{
initlock(&kmem.lock, "kmem");
// init all refcnt to 1, which would later be freed to 0 by kfree()
for(uint64 p = PGROUNDUP((uint64)end); p + PGSIZE <= PHYSTOP; p += PGSIZE)
phypg_refcnt[p/PGSIZE] = 1;
freerange(end, (void*)PHYSTOP);
}
void
freerange(void *pa_start, void *pa_end)
{
char *p;
p = (char*)PGROUNDUP((uint64)pa_start);
for(; p + PGSIZE <= (char*)pa_end; p += PGSIZE)
kfree(p);
}
// Free the page of physical memory pointed at by pa,
// which normally should have been returned by a
// call to kalloc(). (The exception is when
// initializing the allocator; see kinit above.)
void
kfree(void *pa)
{
struct run *r;
if(((uint64)pa % PGSIZE) != 0 || (char*)pa < end || (uint64)pa >= PHYSTOP)
panic("kfree");
refcnt_dec((uint64)pa);
if(phypg_refcnt[(uint64)pa/PGSIZE] > 0)
// We still have refs to this phy page, do not actually free it
return;
// Fill with junk to catch dangling refs.
memset(pa, 1, PGSIZE);
r = (struct run*)pa;
acquire(&kmem.lock);
r->next = kmem.freelist;
kmem.freelist = r;
release(&kmem.lock);
}
// Allocate one 4096-byte page of physical memory.
// Returns a pointer that the kernel can use.
// Returns 0 if the memory cannot be allocated.
void *
kalloc(void)
{
struct run *r;
acquire(&kmem.lock);
r = kmem.freelist;
if(r){
if(phypg_refcnt[(uint64)r/PGSIZE])
panic("kalloc: invalid refcnt");
phypg_refcnt[(uint64)r/PGSIZE] = 1;
kmem.freelist = r->next;
}
release(&kmem.lock);
if(r)
memset((char*)r, 5, PGSIZE); // fill with junk
return (void*)r;
}
int
get_freemem(void)
{
int n;
struct run *r;
acquire(&kmem.lock);
for (n = 0, r = kmem.freelist; r; r = r->next)
n++;
release(&kmem.lock);
return n * PGSIZE;
}
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