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kalloc.c
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// Physical memory allocator, for user processes,
// kernel stacks, page-table pages,
// and pipe buffers. Allocates whole 4096-byte pages.
#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "riscv.h"
#include "defs.h"
void freerange(void *pa_start, void *pa_end);
extern char end[]; // first address after kernel.
// defined by kernel.ld.
struct run
{
struct run *next;
};
struct kmem
{
struct spinlock lock;
struct run *freelist;
} kmems[NCPU];
// NCPU: from kernel/param.h; 8 - maximum number of CPUs
void kinit()
{
for (int i = 0; i < NCPU; i++)
initlock(&kmems[i].lock, "kmems");
freerange(end, (void *)PHYSTOP);
}
void freerange(void *pa_start, void *pa_end)
{
char *p;
p = (char *)PGROUNDUP((uint64)pa_start);
for (; p + PGSIZE <= (char *)pa_end; p += PGSIZE)
kfree(p);
}
// 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(void *pa)
{
struct run *r;
if (((uint64)pa % PGSIZE) != 0 || (char *)pa < end || (uint64)pa >= PHYSTOP)
panic("kfree");
// Fill with junk to catch dangling refs.
memset(pa, 1, PGSIZE);
r = (struct run *)pa;
push_off();
{
int id = cpuid();
acquire(&kmems[id].lock);
{
r->next = kmems[id].freelist;
kmems[id].freelist = r;
}
release(&kmems[id].lock);
}
pop_off();
}
// Allocate one 4096-byte page of physical memory.
// Returns a pointer that the kernel can use.
// Returns 0 if the memory cannot be allocated.
void *kalloc(void)
{
struct run *r;
push_off();
{
int id = cpuid();
// check id’s(self) freelist first
acquire(&kmems[id].lock);
{
r = kmems[id].freelist;
if (r)
kmems[id].freelist = r->next;
}
release(&kmems[id].lock);
// r = null, then steal from others' freelist
for (int i = 0; (!r) && (i < NCPU); i++)
{
if (i == id)
continue;
acquire(&kmems[i].lock);
{
r = kmems[i].freelist;
if (r)
kmems[i].freelist = r->next;
}
release(&kmems[i].lock);
}
}
pop_off();
if (r)
memset((char *)r, 5, PGSIZE); // fill with junk
return (void *)r;
}