Linux高端內存映射(中)

臨時內核映射

臨時內核映射和永久內核映射相比，其最大的特點就是不會阻塞請求映射頁框的進程，因此臨時內核映射請求可以發生在中斷和可延遲函數中。系統中的每個CPU都有自己的臨時內核映射窗口，根據不同的需求，選擇不同的窗口來創建映射，這些窗口都以枚舉類型定義在km_type中

1. enum km_type {
2. KMAP_D(0) KM_BOUNCE_READ,
3. KMAP_D(1) KM_SKB_SUNRPC_DATA,
4. KMAP_D(2) KM_SKB_DATA_SOFTIRQ,
5. KMAP_D(3) KM_USER0,
6. KMAP_D(4) KM_USER1,
7. KMAP_D(5) KM_BIO_SRC_IRQ,
8. KMAP_D(6) KM_BIO_DST_IRQ,
9. KMAP_D(7) KM_PTE0,
10. KMAP_D(8) KM_PTE1,
11. KMAP_D(9) KM_IRQ0,
12. KMAP_D(10) KM_IRQ1,
13. KMAP_D(11) KM_SOFTIRQ0,
14. KMAP_D(12) KM_SOFTIRQ1,
15. KMAP_D(13) KM_SYNC_ICACHE,
16. KMAP_D(14) KM_SYNC_DCACHE,
17. /* UML specific, for copy_*_user - used in do_op_one_page */
18. KMAP_D(15) KM_UML_USERCOPY,
19. KMAP_D(16) KM_IRQ_PTE,
20. KMAP_D(17) KM_NMI,
21. KMAP_D(18) KM_NMI_PTE,
22. KMAP_D(19) KM_TYPE_NR
23. };

其中KM_TYPE_NR標志了一個CPU可以擁有多少個頁表項窗口來建立映射。

臨時內核映射的實現也比永久內核映射要簡單，當一個進程申請在某個窗口創建映射，即使這個窗口已經在之前就建立了映射，新的映射也會建立並且覆蓋之前的映射，所以說這種映射機制是臨時的，並且不會阻塞當前進程。

1. void *kmap_atomic(struct page *page, enum km_type type)
2. {
3. return kmap_atomic_prot(page, type, kmap_prot);
4. }

1. /*
2. * kmap_atomic/kunmap_atomic is significantly faster than kmap/kunmap because
3. * no global lock is needed and because the kmap code must perform a global TLB
4. * invalidation when the kmap pool wraps.
5. *
6. * However when holding an atomic kmap it is not legal to sleep, so atomic
7. * kmaps are appropriate for short, tight code paths only.
8. */
9. void *kmap_atomic_prot(struct page *page, enum km_type type, pgprot_t prot)
10. {
11. enum fixed_addresses idx;
12. unsigned long vaddr;
14. /* even !CONFIG_PREEMPT needs this, for in_atomic in do_page_fault */
15. /*為了保證函數的原子性，禁止page fault handler*/
16. pagefault_disable();
18. if (!PageHighMem(page))/*屬於低端內存則直接返回page的線性地址*/
19. return page_address(page);
21. debug_kmap_atomic(type);
23. /*smp_processor_id()得到CPU的標識號，用KM_TYPE_NR乘以該標識號就得到了該CPU可用窗口的區段，
24. 再加上type就得到了相應的屬於該CPU的窗口*/
25. idx = type + KM_TYPE_NR*smp_processor_id();
26. vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);/*得到對應頁表項的虛擬地址*/
27. BUG_ON(!pte_none(*(kmap_pte-idx)));
28. /*將頁表項與page進行關聯，用kmap_pte-idx而不是用kmap_pte+idx,因為固定映射區是逆向生長的，
29. 也就是說枚舉項越靠前的部分的虛擬地址越靠後*/
30. set_pte(kmap_pte-idx, mk_pte(page, prot));
32. return (void *)vaddr;
33. }

若要手動撤銷當前的臨時內核映射，則可調用kunmap_atomic()函數

1. void kunmap_atomic(void *kvaddr, enum km_type type)
2. {
3. unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;
4. enum fixed_addresses idx = type + KM_TYPE_NR*smp_processor_id();
6. /*
7. * Force other mappings to Oops if they'll try to access this pte
8. * without first remap it. Keeping stale mappings around is a bad idea
9. * also, in case the page changes cacheability attributes or becomes
10. * a protected page in a hypervisor.
11. */
12. /*如果建立了映射則清除頁表項的內容，並且淸刷相應的TLB項*/
13. if (vaddr == __fix_to_virt(FIX_KMAP_BEGIN+idx))
14. kpte_clear_flush(kmap_pte-idx, vaddr);
15. else {
16. #ifdef CONFIG_DEBUG_HIGHMEM
17. BUG_ON(vaddr < PAGE_OFFSET);
18. BUG_ON(vaddr >= (unsigned long)high_memory);
19. #endif
20. }
22. pagefault_enable();/*重新使能page fault handler*/
23. }

相關閱讀：

Linux高端內存映射(上) http://www.linuxidc.com/Linux/2012-05/60627.htm
Linux高端內存映射(中) http://www.linuxidc.com/Linux/2012-05/60628.htm
Linux高端內存映射(中) http://www.linuxidc.com/Linux/2012-05/60902.htm