xv6-riscv-kernel/kernel/kalloc.c

// Physical memory allocator, for user processes,
// kernel stacks, page-table pages,
// and pipe buffers. Allocates whole 4096-byte pages.

#include "types.h"
#include "memlayout.h"
#include "spinlock.h"
#include "riscv.h"
#include "defs.h"

/** Free list of physical pages. */
void freerange(void *pa_start, void *pa_end);

/** first address after kernel. defined by kernel.ld. */
extern char end[];

/** A run is a node in the free list. */
struct run {
  struct run *next;
};

/** Kernel memory allocator. */
struct {
  struct spinlock lock;
  struct run     *freelist;
} kmem;

void
kinit()
{
  initlock(&kmem.lock, "kmem");
  freerange(end, (void *)PHYSTOP);
}

void
freerange(void *pa_start, void *pa_end)
{
  char *p;
  p = (char *)PGROUNDUP((u64)pa_start);
  for(; p + PGSIZE <= (char *)pa_end; p += PGSIZE)
    kfree(p);
}

void
kfree(void *pa)
{
  struct run *r;

  // Assert that page is a ligned to a page boundary and that its correctly sized
  if(((u64)pa % PGSIZE) != 0 || (char *)pa < end || (u64)pa >= PHYSTOP)
    panic("kfree");

  // 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);
}

void *
kalloc(void)
{
  struct run *r;

  acquire(&kmem.lock);
  r = kmem.freelist;
  if(r)
    kmem.freelist = r->next;
  release(&kmem.lock);

  if(r)
    memset((char *)r, 5, PGSIZE); // fill with junk
  return (void *)r;
}