116 lines
2.7 KiB
C
116 lines
2.7 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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// One reason the page size is 4k is that the x86 segment size
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// granularity is 4k.
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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 "spinlock.h"
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struct spinlock kalloc_lock;
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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 run *freelist;
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// Initialize free list of physical pages.
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// This code cheats by just considering one megabyte of
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// pages after _end. Real systems would determine the
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// amount of memory available in the system and use it all.
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void
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kinit(void)
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{
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extern int end;
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uint mem;
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char *start;
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initlock(&kalloc_lock, "kalloc");
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start = (char*) &end;
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start = (char*) (((uint)start + PAGE) & ~(PAGE-1));
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mem = 256; // assume computer has 256 pages of RAM
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cprintf("mem = %d\n", mem * PAGE);
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kfree(start, mem * PAGE);
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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 % PAGE)
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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(&kalloc_lock);
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p = (struct run*)v;
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pend = (struct run*)(v + len);
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for(rp=&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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panic("freeing free page");
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if(pend == r){ // p next to r: replace r with p
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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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if(rend == p){ // r next to p: replace p with r
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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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}
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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(&kalloc_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 % PAGE || n <= 0)
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panic("kalloc");
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acquire(&kalloc_lock);
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for(rp=&freelist; (r=*rp) != 0; rp=&r->next){
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if(r->len == n){
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*rp = r->next;
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release(&kalloc_lock);
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return (char*)r;
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}
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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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release(&kalloc_lock);
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return p;
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}
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}
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release(&kalloc_lock);
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cprintf("kalloc: out of memory\n");
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return 0;
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}
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