xv6/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.
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#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
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struct run {
struct run *next;
};
struct {
struct spinlock lock;
int use_lock;
struct run *freelist;
} kmem;
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// 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;
}
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void
freerange(void *vstart, void *vend)
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{
char *p;
p = (char*)PGROUNDUP((uint)vstart);
for(; p + PGSIZE <= (char*)vend; p += PGSIZE)
kfree(p);
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}
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//PAGEBREAK: 21
// Free the page of physical memory pointed at by v,
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// which normally should have been returned by a
// call to kalloc(). (The exception is when
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// initializing the allocator; see kinit above.)
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void
kfree(char *v)
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{
struct run *r;
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if((uint)v % PGSIZE || v < end || v2p(v) >= PHYSTOP)
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panic("kfree");
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// 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);
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}
// Allocate one 4096-byte page of physical memory.
// Returns a pointer that the kernel can use.
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// Returns 0 if the memory cannot be allocated.
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char*
kalloc(void)
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{
struct run *r;
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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;
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}