112 lines
2.4 KiB
C
112 lines
2.4 KiB
C
// Physical memory allocator, intended to allocate
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// memory for user processes. Allocates in 4096-byte pages.
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// Free list is kept sorted and combines adjacent pages into
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// long runs, to make it easier to allocate big segments.
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// This combining is not useful now that xv6 uses paging.
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#include "types.h"
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#include "defs.h"
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#include "param.h"
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#include "mmu.h"
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#include "spinlock.h"
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struct run {
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struct run *next;
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int len; // bytes
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};
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struct {
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struct spinlock lock;
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struct run *freelist;
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} kmem;
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int nfreemem;
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// Initialize free list of physical pages.
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void
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kinit(char *p, uint len)
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{
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initlock(&kmem.lock, "kmem");
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nfreemem = 0;
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kfree(p, len);
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}
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// Free the len bytes of memory pointed at by v,
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// which normally should have been returned by a
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// call to kalloc(len). (The exception is when
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// initializing the allocator; see kinit above.)
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void
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kfree(char *v, int len)
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{
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struct run *r, *rend, **rp, *p, *pend;
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if(len <= 0 || len % PGSIZE)
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panic("kfree");
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// Fill with junk to catch dangling refs.
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memset(v, 1, len);
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acquire(&kmem.lock);
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nfreemem += len;
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p = (struct run*)v;
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pend = (struct run*)(v + len);
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for(rp=&kmem.freelist; (r=*rp) != 0 && r <= pend; rp=&r->next){
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rend = (struct run*)((char*)r + r->len);
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if(r <= p && p < rend) {
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cprintf("freeing a free page: r = 0x%x p = 0x%x rend = 0x%x\n",
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r, p, rend);
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panic("freeing free page");
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}
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if(rend == p){ // r before p: expand r to include p
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r->len += len;
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if(r->next && r->next == pend){ // r now next to r->next?
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r->len += r->next->len;
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r->next = r->next->next;
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}
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goto out;
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}
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if(pend == r){ // p before r: expand p to include, replace r
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p->len = len + r->len;
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p->next = r->next;
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*rp = p;
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goto out;
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}
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}
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// Insert p before r in list.
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p->len = len;
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p->next = r;
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*rp = p;
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out:
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release(&kmem.lock);
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}
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// Allocate n bytes of physical memory.
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// Returns a kernel-segment pointer.
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// Returns 0 if the memory cannot be allocated.
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char*
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kalloc(int n)
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{
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char *p;
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struct run *r, **rp;
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if(n % PGSIZE || n <= 0)
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panic("kalloc");
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acquire(&kmem.lock);
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for(rp=&kmem.freelist; (r=*rp) != 0; rp=&r->next){
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if(r->len >= n){
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r->len -= n;
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p = (char*)r + r->len;
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if(r->len == 0)
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*rp = r->next;
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nfreemem -= n;
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release(&kmem.lock);
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return p;
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}
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}
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release(&kmem.lock);
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return 0;
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}
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