// 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;
}