Linux物理內存管理區初始化

Linux物理內存管理區在start_kernel函數中進行初始化，此時啟動分配器已經建立，所以可以從bootmem中分配需要的內存。

一、全局變量初始化

max_pfn：最大物理頁面幀號

start_kernel()->setup_arch()->e820_end_of_ram_pfn()找出最大可用內存頁面幀號。

1. void __init setup_arch(char **cmdline_p)
2. {
3. ……
4. /*
5. * partially used pages are not usable - thus
6. * we are rounding upwards:
7. */
8. /*遍歷e820.map，找到系統中得最大內存數，
9. 這個內存數需小於4G*/
10. max_pfn = e820_end_of_ram_pfn();
11. ……
12. ｝

1. unsigned long __init e820_end_of_ram_pfn(void)
2. {
3. /*MAX_ARCH_PFN為4G空間*/
4. return e820_end_pfn(MAX_ARCH_PFN, E820_RAM);
5. }

1. /*
2. * Find the highest page frame number we have available
3. */
4. static unsigned long __init e820_end_pfn(unsigned long limit_pfn, unsigned type)
5. {
6. int i;
7. unsigned long last_pfn = 0;
8. unsigned long max_arch_pfn = MAX_ARCH_PFN;/*4G地址空間對應的頁面數*/
10. /*對e820中所有的內存塊,其中e820為從bios中探測到的頁面數存放處*/
11. for (i = 0; i < e820.nr_map; i++) {
12. struct e820entry *ei = &e820.map[i];/*第i個物理頁面塊*/
13. unsigned long start_pfn;
14. unsigned long end_pfn;
16. if (ei->type != type)/*與找的類型不匹配*/
17. continue;
18. /*起始地址對應的頁面幀號*/
19. start_pfn = ei->addr >> PAGE_SHIFT;
20. /*結束物理地址對應的頁面幀號*/
21. end_pfn = (ei->addr + ei->size) >> PAGE_SHIFT;
23. if (start_pfn >= limit_pfn)
24. continue;
25. if (end_pfn > limit_pfn) {
26. last_pfn = limit_pfn;/*找到的結束頁面幀號大於限制大小時*/
27. break;
28. }
29. if (end_pfn > last_pfn)
30. last_pfn = end_pfn;/*保存更新last_pfn*/
31. }
33. if (last_pfn > max_arch_pfn)/*大於4G空間時*/
34. last_pfn = max_arch_pfn;
35. /*打印輸出信息*/
36. printk(KERN_INFO "last_pfn = %#lx max_arch_pfn = %#lx\n",
37. last_pfn, max_arch_pfn);
38. /*返回最後一個頁面幀號*/
39. return last_pfn;
40. }

max_low_pfn：低端內存最大頁面數

start_kernel()->setup_arch()->find_low_pfn_range()

1. /*
2. * Determine low and high memory ranges:
3. */
4. /*找到低端內存的做大內存頁面數，初始化兩個變量*/
5. void __init find_low_pfn_range(void)
6. {
7. /* it could update max_pfn */
8. /*當內存的大小本來就小於低端內存的做大頁框數時；
9. 直接沒有高端地址映射*/
10. if (max_pfn <= MAXMEM_PFN)
11. lowmem_pfn_init();
12. else/*這是一般PC機的運行流程，存在高端映射*/
13. highmem_pfn_init();
14. }

我們直接看具有高端地址空間的部分。

1. /*
2. * We have more RAM than fits into lowmem - we try to put it into
3. * highmem, also taking the highmem=x boot parameter into account:
4. */
5. /*高端地址空間的頁面數可以在啟動中進行配置；
6. 如果不配置，在這裡進行設置大小*/
7. void __init highmem_pfn_init(void)
8. {
9. /*MAXMEM_PFN為最大物理地址-(4M+4M+8K+128M);
10. 所以低端內存的大小其實比我們說的896M低一些*/
11. max_low_pfn = MAXMEM_PFN;
13. if (highmem_pages == -1)/*高端內存頁面數如果在開機沒有設置*/
14. highmem_pages = max_pfn - MAXMEM_PFN;/*總頁面數減去低端頁面數*/
15. /*如果highmem_pages變量在啟動項設置了，那麼在這裡就要進行這樣的判斷，因為可能出現不一致的情況*/
16. if (highmem_pages + MAXMEM_PFN < max_pfn)
17. max_pfn = MAXMEM_PFN + highmem_pages;
19. if (highmem_pages + MAXMEM_PFN > max_pfn) {
20. printk(KERN_WARNING MSG_HIGHMEM_TOO_SMALL,
21. pages_to_mb(max_pfn - MAXMEM_PFN),
22. pages_to_mb(highmem_pages));
23. highmem_pages = 0;
24. }
25. #ifndef CONFIG_HIGHMEM
26. /* Maximum memory usable is what is directly addressable */
27. printk(KERN_WARNING "Warning only %ldMB will be used.\n", MAXMEM>>20);
28. if (max_pfn > MAX_NONPAE_PFN)
29. printk(KERN_WARNING "Use a HIGHMEM64G enabled kernel.\n");
30. else
31. printk(KERN_WARNING "Use a HIGHMEM enabled kernel.\n");
32. max_pfn = MAXMEM_PFN;
33. #else /* !CONFIG_HIGHMEM *//*存在高端地址情況*/
34. #ifndef CONFIG_HIGHMEM64G
35. /*在沒有配置64G的情況下，內存的大小不能超過4G*/
36. if (max_pfn > MAX_NONPAE_PFN) {
37. max_pfn = MAX_NONPAE_PFN;
38. printk(KERN_WARNING MSG_HIGHMEM_TRIMMED);
39. }
40. #endif /* !CONFIG_HIGHMEM64G */
41. #endif /* !CONFIG_HIGHMEM */
42. }