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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"
struct run {
struct run *next;
};
struct {
struct spinlock lock;
struct run *freelist;
} kmem;
extern char end[]; // first address after kernel loaded from ELF file
static char *newend;
// A simple page allocator to get off the ground during entry
char *
enter_alloc(void)
{
if (newend == 0)
newend = end;
if ((uint) newend >= KERNBASE + 0x400000)
panic("only first 4Mbyte are mapped during entry");
void *p = (void*)PGROUNDUP((uint)newend);
memset(p, 0, PGSIZE);
newend = newend + PGSIZE;
return p;
}
// Initialize free list of physical pages.
void
kinit(void)
{
char *p;
initlock(&kmem.lock, "kmem");
p = (char*)PGROUNDUP((uint)newend);
for(; p + PGSIZE <= (char*)p2v(PHYSTOP); 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);
acquire(&kmem.lock);
r = (struct run*)v;
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.
char*
kalloc(void)
{
struct run *r;
acquire(&kmem.lock);
r = kmem.freelist;
if(r)
kmem.freelist = r->next;
release(&kmem.lock);
return (char*)r;
}
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