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kalloc.c
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kalloc.c
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// 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
// defined by the kernel linker script in kernel.ld
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;
}