内核首先检查待释放内存区域是不是在高端内存区域

• 如果内存区域在普通内存区, 则内核并没有通过kmap_high对其建立持久的内核映射, 当然也无需用kunmap_high释放
• 如果内存区域在高端内存区, 则内核通过kunmap_high释放该内存空间

2.4.2 kunmap_high函数

kunmap_high函数定义在mm/highmem.c?v=4.7, line 328

#ifdef CONFIG_HIGHMEM
/**
 * kunmap_high - unmap 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;
    unsigned int color = get_pkmap_color(page);
    wait_queue_head_t *pkmap_map_wait;

    lock_kmap_any(flags);
    vaddr = (unsigned long)page_address(page);
    BUG_ON(!vaddr);
    nr = PKMAP_NR(vaddr);   /*永久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.
         */
        pkmap_map_wait = get_pkmap_wait_queue_head(color);
        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);
}

EXPORT_SYMBOL(kunmap_high);
#endif

3 临时内核映射

刚才描述的kmap函数不能用于中断处理程序, 因为它可能进入睡眠状态. 如果pkmap数组中没有空闲位置, 该函数会进入睡眠状态, 直至情形有所改善.

void *kmap_atomic(struct page *page)
{
    unsigned int idx;
    unsigned long vaddr;
    void *kmap;
    int type;

    preempt_disable();
    pagefault_disable();
    if (!PageHighMem(page))
        return page_address(page);

#ifdef CONFIG_DEBUG_HIGHMEM
    /*
     * There is no cache coherency issue when non VIVT, so force the
     * dedicated kmap usage for better debugging purposes in that case.
     */
    if (!cache_is_vivt())
        kmap = NULL;
    else
#endif
        kmap = kmap_high_get(page);
    if (kmap)
        return kmap;

    type = kmap_atomic_idx_push();

    idx = FIX_KMAP_BEGIN + type + KM_TYPE_NR * smp_processor_id();
    vaddr = __fix_to_virt(idx);
#ifdef CONFIG_DEBUG_HIGHMEM
    /*
     * With debugging enabled, kunmap_atomic forces that entry to 0.
     * Make sure it was indeed properly unmapped.
     */
    BUG_ON(!pte_none(get_fixmap_pte(vaddr)));
#endif
    /*
     * When debugging is off, kunmap_atomic leaves the previous mapping
     * in place, so the contained TLB flush ensures the TLB is updated
     * with the new mapping.
     */
    set_fixmap_pte(idx, mk_pte(page, kmap_prot));

    return (void *)vaddr;
}
EXPORT_SYMBOL(kmap_atomic);

这个函数不会被阻塞, 因此可以用在中断上下文和起亚不能重新调度的地方. 它也禁止内核抢占, 这是有必要的, 因此映射对每个处理器都是唯一的(调度可能对哪个处理器执行哪个进程做变动).

3.2 kunmap_atomic函数

可以通过函数kunmap_atomic取消映射

/*
 * Prevent people trying to call kunmap_atomic() as if it were kunmap()
 * kunmap_atomic() should get the return value of kmap_atomic, not the page.
 */
#define kunmap_atomic(addr)                     \
do {                                \
    BUILD_BUG_ON(__same_type((addr), struct page *));       \
    __kunmap_atomic(addr);                  \
} while (0)

这个函数也不会阻塞. 在很多体系结构中, 除非激活了内核抢占, 否则kunmap_atomic根本无事可做, 因为只有在下一个临时映射到来前上一个临时映射才有效. 因此, 内核完全可以”忘掉”kmap_atomic映射, kunmap_atomic也无需做什么实际的事情. 下一个原子映射将自动覆盖前一个映射.