Linux 分页实现

内存分配

void *malloc(unsigned int len)
{
   struct _bucket_dir *bdir;
   struct bucket_desc *bdesc;
   void         *retval;

   /*
    * First we search the bucket_dir to find the right bucket change
    * for this request.
    */
   for (bdir = bucket_dir; bdir->size; bdir++)
      if (bdir->size >= len)
         break;
   if (!bdir->size) {
      printk("malloc called with impossibly large argument (%d)\n",
         len);
      panic("malloc: bad arg");
   }
   /*
    * Now we search for a bucket descriptor which has free space
    */
   cli(); /* Avoid race conditions */
   for (bdesc = bdir->chain; bdesc; bdesc = bdesc->next) 
      if (bdesc->freeptr)
         break;
   /*
    * If we didn't find a bucket with free space, then we'll 
    * allocate a new one.
    */
   if (!bdesc) {
      char      *cp;
      int       i;

      if (!free_bucket_desc) 
         init_bucket_desc();
      bdesc = free_bucket_desc;
      free_bucket_desc = bdesc->next;
      bdesc->refcnt = 0;
      bdesc->bucket_size = bdir->size;
      bdesc->page = bdesc->freeptr = (void *) cp = get_free_page();
      if (!cp)
         panic("Out of memory in kernel malloc()");
      /* Set up the chain of free objects */
      for (i=PAGE_SIZE/bdir->size; i > 1; i--) {
         *((char **) cp) = cp + bdir->size;
         cp += bdir->size;
      }
      *((char **) cp) = 0;
      bdesc->next = bdir->chain; /* OK, link it in! */
      bdir->chain = bdesc;
   }
   retval = (void *) bdesc->freeptr;
   bdesc->freeptr = *((void **) retval);
   bdesc->refcnt++;
   sti(); /* OK, we're safe again */
   return(retval);
}

/* these are not to be changed without changing head.s etc */
#define LOW_MEM 0x100000
#define PAGING_MEMORY (15*1024*1024)
#define PAGING_PAGES (PAGING_MEMORY>>12)
#define MAP_NR(addr) (((addr)-LOW_MEM)>>12)
#define USED 100

static unsigned char mem_map [ PAGING_PAGES ] = {0,};

unsigned long get_free_page(void)
{
register unsigned long __res asm("ax");

__asm__("std ; repne ; scasb\n\t"
   "jne 1f\n\t"
   "movb $1,1(%%edi)\n\t"
   "sall $12,%%ecx\n\t"
   "addl %2,%%ecx\n\t"
   "movl %%ecx,%%edx\n\t"
   "movl $1024,%%ecx\n\t"
   "leal 4092(%%edx),%%edi\n\t"
   "rep ; stosl\n\t"
   "movl %%edx,%%eax\n"
   "1:"
   :"=a" (__res)
   :"0" (0),"i" (LOW_MEM),"c" (PAGING_PAGES),
   "D" (mem_map+PAGING_PAGES-1)
   :"di","cx","dx");
return __res;
}

#define copy_page(from,to) \
__asm__("cld ; rep ; movsl"::"S" (from),"D" (to),"c" (1024):"cx","di","si")