Linux高端内存管理之内核映射

　　kunmap()函数撤销先前由kmap()建立的内核映射
　　如果页确实在高端内存中，则调用kunmap_high()函数
* kunmap_high - map a highmem page into memory
* @page: &struct page to unmap
*
* If ARCH_NEEDS_KMAP_HIGH_GET is not defined then this may be called
* only from user context.
*/
void kunmap_high(struct page *page)
{
unsigned long vaddr;
unsigned long nr;
unsigned long flags;
int need_wakeup;
lock_kmap_any(flags);
vaddr = (unsigned long)page_address(page);
BUG_ON(!vaddr);
nr = PKMAP_NR(vaddr);/*内存区域开始的第几个页面*/
/*
* A count must never go down to zero
* without a TLB flush!
*/
need_wakeup = 0;
switch (--pkmap_count[nr]) {/*减小这个值，因为在映射的时候对其进行了加2*/
case 0:
BUG();
case 1:
/*
* Avoid an unnecessary wake_up() function call.
* The common case is pkmap_count[] == 1， but
* no waiters.
* The tasks queued in the wait-queue are guarded
* by both the lock in the wait-queue-head and by
* the kmap_lock.  As the kmap_lock is held here，
* no need for the wait-queue-head's lock.  Simply
* test if the queue is empty.
*/
need_wakeup = waitqueue_active(&pkmap_map_wait);
}
unlock_kmap_any(flags);
/* do wake-up， if needed， race-free outside of the spin lock */
if (need_wakeup)
wake_up(&pkmap_map_wait);
}