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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"
// NOTE: leave interrupts disabled to avoid deadlocks & race conditions when using this macro!!!
#define CUR_KMEM (kmem_list[cpuid()])
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 kmem {
struct spinlock lock;
struct run *freelist;
};
struct kmem kmem_list[NCPU];
int phypg_refcnt[PHYSTOP/PGSIZE];
struct spinlock refcnt_lock;
// Increase the refcnt
int
refcnt_inc(uint64 pa)
{
acquire(&refcnt_lock);
int *prefcnt = &phypg_refcnt[pa/PGSIZE];
if(pa > PHYSTOP || *prefcnt < 1)
panic("increase refcnt");
(*prefcnt)++;
release(&refcnt_lock);
return *prefcnt;
}
// Decrease the refcnt
int
refcnt_dec(uint64 pa)
{
acquire(&refcnt_lock);
int *prefcnt = &phypg_refcnt[pa/PGSIZE];
if(pa > PHYSTOP || *prefcnt < 1)
panic("decrease refcnt");
(*prefcnt)--;
release(&refcnt_lock);
return *prefcnt;
}
void
kinit()
{
for(int i = 0; i < NCPU; i++){
static char lock_name[8];
snprintf(lock_name, sizeof(lock_name), "kmem.%d", i);
initlock(&kmem_list[i].lock, lock_name);
}
// 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;
initlock(&refcnt_lock, "refcnt");
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;
push_off();
struct kmem *kmem = &CUR_KMEM;
acquire(&kmem->lock);
r->next = kmem->freelist;
kmem->freelist = r;
release(&kmem->lock);
pop_off();
}
// 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;
push_off();
struct kmem *kmem = &CUR_KMEM;
acquire(&kmem->lock);
r = kmem->freelist;
if(r){
acquire(&refcnt_lock);
if(phypg_refcnt[(uint64)r/PGSIZE])
panic("kalloc: invalid refcnt");
phypg_refcnt[(uint64)r/PGSIZE] = 1;
release(&refcnt_lock);
kmem->freelist = r->next;
}
// release the origin lock to avoid deadlocks
release(&kmem->lock);
if(!r){
// try to steal mem from other cpu's kmem
for(int i = 0; i < NCPU; i++){
if(kmem == &kmem_list[i])
continue;
acquire(&kmem_list[i].lock);
struct run *f = kmem_list[i].freelist;
if(f){
r = f;
kmem_list[i].freelist = f->next;
}
if(r){
// acquire the refcnt lock to set refcnt
// lock is a must to prevent refcnt races
acquire(&refcnt_lock);
// release previous lock now
release(&kmem_list[i].lock);
if(phypg_refcnt[(uint64)r/PGSIZE])
panic("kalloc: invalid refcnt");
phypg_refcnt[(uint64)r/PGSIZE] = 1;
release(&refcnt_lock);
break;
}
release(&kmem_list[i].lock);
}
}
if(r)
memset((char*)r, 5, PGSIZE); // fill with junk
pop_off();
return (void*)r;
}
int
get_freemem(void)
{
int n;
struct run *r;
for(int i = 0; i < NCPU; i++){
acquire(&kmem_list[i].lock);
for(n = 0, r = kmem_list[i].freelist; r; r = r->next)
n++;
release(&kmem_list[i].lock);
}
return n * PGSIZE;
}
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