kalloc.c

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

#include "types.h"
#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "mmu.h"
#include "spinlock.h"

void freerange(void *vstart, void *vend);

extern char end[]; // first address after kernel loaded from ELF file

struct run {
    struct run *next;
};

struct {
    struct spinlock lock;
    int use_lock;
    struct run *freelist;
} kmem;

// Initialization happens in two phases.
// 1. main() calls kinit1() while still using entrypgdir to place just
// the pages mapped by entrypgdir on free list.
// 2. main() calls kinit2() with the rest of the physical pages
// after installing a full page table that maps them on all cores.
void
kinit1(void *vstart, void *vend) {
    initlock(&kmem.lock, "kmem");
    kmem.use_lock = 0;
    freerange(vstart, vend);
}

void
kinit2(void *vstart, void *vend) {
    freerange(vstart, vend);
    kmem.use_lock = 1;
}

void
freerange(void *vstart, void *vend) {
    char *p;
    p = (char *) PGROUNDUP((uint) vstart);
    for (; p + PGSIZE <= (char *) vend; p += PGSIZE)
        kfree(p);
}

//PAGEBREAK: 21
// Free the page of physical memory pointed at by v,
// which normally should have been returned by a
// call to kalloc().  (The exception is when
// initializing the allocator; see kinit above.)
void
kfree(char *v) {
    struct run *r;

    if ((uint) v % PGSIZE || v < end || v2p(v) >= PHYSTOP)
        panic("kfree");

    // Fill with junk to catch dangling refs.
    memset(v, 1, PGSIZE);

    if (kmem.use_lock)
        acquire(&kmem.lock);
    r = (struct run *) v;
    r->next = kmem.freelist;
    kmem.freelist = r;
    if (kmem.use_lock)
        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.
char *
kalloc(void) {
    struct run *r;

    if (kmem.use_lock)
        acquire(&kmem.lock);
    r = kmem.freelist;
    if (r)
        kmem.freelist = r->next;
    if (kmem.use_lock)
        release(&kmem.lock);
    return (char *) r;
}