Linux-3.14.12内存管理笔记【建立内核页表（2）】-低端内存的建立
- 2019 年 10 月 11 日
- 筆記
前面的前奏已经分析介绍了建立内核页表相关变量的设置准备，接下来转入正题分析内核页表的建立。
建立内核页表的关键函数init_mem_mapping()：
【file：/arch/x86/mm/init.c】 void __init init_mem_mapping(void) { unsigned long end; probe_page_size_mask(); #ifdef CONFIG_X86_64 end = max_pfn << PAGE_SHIFT; #else end = max_low_pfn << PAGE_SHIFT; #endif /* the ISA range is always mapped regardless of memory holes */ init_memory_mapping(0, ISA_END_ADDRESS); /* * If the allocation is in bottom-up direction, we setup direct mapping * in bottom-up, otherwise we setup direct mapping in top-down. */ if (memblock_bottom_up()) { unsigned long kernel_end = __pa_symbol(_end); /* * we need two separate calls here. This is because we want to * allocate page tables above the kernel. So we first map * [kernel_end, end) to make memory above the kernel be mapped * as soon as possible. And then use page tables allocated above * the kernel to map [ISA_END_ADDRESS, kernel_end). */ memory_map_bottom_up(kernel_end, end); memory_map_bottom_up(ISA_END_ADDRESS, kernel_end); } else { memory_map_top_down(ISA_END_ADDRESS, end); } #ifdef CONFIG_X86_64 if (max_pfn > max_low_pfn) { /* can we preseve max_low_pfn ?*/ max_low_pfn = max_pfn; } #else early_ioremap_page_table_range_init(); #endif load_cr3(swapper_pg_dir); __flush_tlb_all(); early_memtest(0, max_pfn_mapped << PAGE_SHIFT); }
其中probe_page_size_mask()实现：
【file：/arch/x86/mm/init.c】 static void __init probe_page_size_mask(void) { init_gbpages(); #if !defined(CONFIG_DEBUG_PAGEALLOC) && !defined(CONFIG_KMEMCHECK) /* * For CONFIG_DEBUG_PAGEALLOC, identity mapping will use small pages. * This will simplify cpa(), which otherwise needs to support splitting * large pages into small in interrupt context, etc. */ if (direct_gbpages) page_size_mask |= 1 << PG_LEVEL_1G; if (cpu_has_pse) page_size_mask |= 1 << PG_LEVEL_2M; #endif /* Enable PSE if available */ if (cpu_has_pse) set_in_cr4(X86_CR4_PSE); /* Enable PGE if available */ if (cpu_has_pge) { set_in_cr4(X86_CR4_PGE); __supported_pte_mask |= _PAGE_GLOBAL; } }
probe_page_size_mask()主要作用是初始化直接映射变量（在init_gbpages()里面）和对page_size_mask变量进行设置，以及根据配置来控制CR4寄存器的置位，用于后面分页时的页面大小情况判定。
回到init_mem_mapping()继续往下走，接着是init_memory_mapping()，其中入参ISA_END_ADDRESS表示ISA总线上设备的地址末尾。
init_mem_mapping()实现：
【file：/arch/x86/mm/init.c】 /* * Setup the direct mapping of the physical memory at PAGE_OFFSET. * This runs before bootmem is initialized and gets pages directly from * the physical memory. To access them they are temporarily mapped. */ unsigned long __init_refok init_memory_mapping(unsigned long start, unsigned long end) { struct map_range mr[NR_RANGE_MR]; unsigned long ret = 0; int nr_range, i; pr_info("init_memory_mapping: [mem %#010lx-%#010lx]n", start, end - 1); memset(mr, 0, sizeof(mr)); nr_range = split_mem_range(mr, 0, start, end); for (i = 0; i < nr_range; i++) ret = kernel_physical_mapping_init(mr[i].start, mr[i].end, mr[i].page_size_mask); add_pfn_range_mapped(start >> PAGE_SHIFT, ret >> PAGE_SHIFT); return ret >> PAGE_SHIFT; }
init_mem_mapping()里面关键操作有三个split_mem_range()、kernel_physical_mapping_init()和add_pfn_range_mapped()函数。
首先分析一下split_mem_range()：
【file：/arch/x86/mm/init.c】 static int __meminit split_mem_range(struct map_range *mr, int nr_range, unsigned long start, unsigned long end) { unsigned long start_pfn, end_pfn, limit_pfn; unsigned long pfn; int i; limit_pfn = PFN_DOWN(end); /* head if not big page alignment ? */ pfn = start_pfn = PFN_DOWN(start); #ifdef CONFIG_X86_32 /* * Don't use a large page for the first 2/4MB of memory * because there are often fixed size MTRRs in there * and overlapping MTRRs into large pages can cause * slowdowns. */ if (pfn == 0) end_pfn = PFN_DOWN(PMD_SIZE); else end_pfn = round_up(pfn, PFN_DOWN(PMD_SIZE)); #else /* CONFIG_X86_64 */ end_pfn = round_up(pfn, PFN_DOWN(PMD_SIZE)); #endif if (end_pfn > limit_pfn) end_pfn = limit_pfn; if (start_pfn < end_pfn) { nr_range = save_mr(mr, nr_range, start_pfn, end_pfn, 0); pfn = end_pfn; } /* big page (2M) range */ start_pfn = round_up(pfn, PFN_DOWN(PMD_SIZE)); #ifdef CONFIG_X86_32 end_pfn = round_down(limit_pfn, PFN_DOWN(PMD_SIZE)); #else /* CONFIG_X86_64 */ end_pfn = round_up(pfn, PFN_DOWN(PUD_SIZE)); if (end_pfn > round_down(limit_pfn, PFN_DOWN(PMD_SIZE))) end_pfn = round_down(limit_pfn, PFN_DOWN(PMD_SIZE)); #endif if (start_pfn < end_pfn) { nr_range = save_mr(mr, nr_range, start_pfn, end_pfn, page_size_mask & (1<<PG_LEVEL_2M)); pfn = end_pfn; } #ifdef CONFIG_X86_64 /* big page (1G) range */ start_pfn = round_up(pfn, PFN_DOWN(PUD_SIZE)); end_pfn = round_down(limit_pfn, PFN_DOWN(PUD_SIZE)); if (start_pfn < end_pfn) { nr_range = save_mr(mr, nr_range, start_pfn, end_pfn, page_size_mask & ((1<<PG_LEVEL_2M)|(1<<PG_LEVEL_1G))); pfn = end_pfn; } /* tail is not big page (1G) alignment */ start_pfn = round_up(pfn, PFN_DOWN(PMD_SIZE)); end_pfn = round_down(limit_pfn, PFN_DOWN(PMD_SIZE)); if (start_pfn < end_pfn) { nr_range = save_mr(mr, nr_range, start_pfn, end_pfn, page_size_mask & (1<<PG_LEVEL_2M)); pfn = end_pfn; } #endif /* tail is not big page (2M) alignment */ start_pfn = pfn; end_pfn = limit_pfn; nr_range = save_mr(mr, nr_range, start_pfn, end_pfn, 0); if (!after_bootmem) adjust_range_page_size_mask(mr, nr_range); /* try to merge same page size and continuous */ for (i = 0; nr_range > 1 && i < nr_range - 1; i++) { unsigned long old_start; if (mr[i].end != mr[i+1].start || mr[i].page_size_mask != mr[i+1].page_size_mask) continue; /* move it */ old_start = mr[i].start; memmove(&mr[i], &mr[i+1], (nr_range - 1 - i) * sizeof(struct map_range)); mr[i--].start = old_start; nr_range--; } for (i = 0; i < nr_range; i++) printk(KERN_DEBUG " [mem %#010lx-%#010lx] page %sn", mr[i].start, mr[i].end - 1, (mr[i].page_size_mask & (1<<PG_LEVEL_1G))?"1G":( (mr[i].page_size_mask & (1<<PG_LEVEL_2M))?"2M":"4k")); return nr_range; }
split_mem_range()根据传入的内存start和end做四舍五入的对齐操作（即round_up和round_down），并根据对齐的情况，把开始、末尾的不对齐部分及中间部分分成了三段，使用save_mr()将其存放在init_mem_mapping()的局部变量数组mr中。划分开来主要是为了允许各部分可以映射不同页面大小，然后如果各划分开来的部分是连续的，映射页面大小也是一致的，则将其合并。最后将映射的情况打印出来，在shell上使用dmesg命令可以看到该打印信息，样例：
接下来看kernel_physical_mapping_init():
【file：/arch/x86/mm/init.c】 /* * This maps the physical memory to kernel virtual address space, a total * of max_low_pfn pages, by creating page tables starting from address * PAGE_OFFSET: */ unsigned long __init kernel_physical_mapping_init(unsigned long start, unsigned long end, unsigned long page_size_mask) { int use_pse = page_size_mask == (1<<PG_LEVEL_2M); unsigned long last_map_addr = end; unsigned long start_pfn, end_pfn; pgd_t *pgd_base = swapper_pg_dir; int pgd_idx, pmd_idx, pte_ofs; unsigned long pfn; pgd_t *pgd; pmd_t *pmd; pte_t *pte; unsigned pages_2m, pages_4k; int mapping_iter; start_pfn = start >> PAGE_SHIFT; end_pfn = end >> PAGE_SHIFT; /* * First iteration will setup identity mapping using large/small pages * based on use_pse, with other attributes same as set by * the early code in head_32.S * * Second iteration will setup the appropriate attributes (NX, GLOBAL..) * as desired for the kernel identity mapping. * * This two pass mechanism conforms to the TLB app note which says: * * "Software should not write to a paging-structure entry in a way * that would change, for any linear address, both the page size * and either the page frame or attributes." */ mapping_iter = 1; if (!cpu_has_pse) use_pse = 0; repeat: pages_2m = pages_4k = 0; pfn = start_pfn; pgd_idx = pgd_index((pfn<<PAGE_SHIFT) + PAGE_OFFSET); pgd = pgd_base + pgd_idx; for (; pgd_idx < PTRS_PER_PGD; pgd++, pgd_idx++) { pmd = one_md_table_init(pgd); if (pfn >= end_pfn) continue; #ifdef CONFIG_X86_PAE pmd_idx = pmd_index((pfn<<PAGE_SHIFT) + PAGE_OFFSET); pmd += pmd_idx; #else pmd_idx = 0; #endif for (; pmd_idx < PTRS_PER_PMD && pfn < end_pfn; pmd++, pmd_idx++) { unsigned int addr = pfn * PAGE_SIZE + PAGE_OFFSET; /* * Map with big pages if possible, otherwise * create normal page tables: */ if (use_pse) { unsigned int addr2; pgprot_t prot = PAGE_KERNEL_LARGE; /* * first pass will use the same initial * identity mapping attribute + _PAGE_PSE. */ pgprot_t init_prot = __pgprot(PTE_IDENT_ATTR | _PAGE_PSE); pfn &= PMD_MASK >> PAGE_SHIFT; addr2 = (pfn + PTRS_PER_PTE-1) * PAGE_SIZE + PAGE_OFFSET + PAGE_SIZE-1; if (is_kernel_text(addr) || is_kernel_text(addr2)) prot = PAGE_KERNEL_LARGE_EXEC; pages_2m++; if (mapping_iter == 1) set_pmd(pmd, pfn_pmd(pfn, init_prot)); else set_pmd(pmd, pfn_pmd(pfn, prot)); pfn += PTRS_PER_PTE; continue; } pte = one_page_table_init(pmd); pte_ofs = pte_index((pfn<<PAGE_SHIFT) + PAGE_OFFSET); pte += pte_ofs; for (; pte_ofs < PTRS_PER_PTE && pfn < end_pfn; pte++, pfn++, pte_ofs++, addr += PAGE_SIZE) { pgprot_t prot = PAGE_KERNEL; /* * first pass will use the same initial * identity mapping attribute. */ pgprot_t init_prot = __pgprot(PTE_IDENT_ATTR); if (is_kernel_text(addr)) prot = PAGE_KERNEL_EXEC; pages_4k++; if (mapping_iter == 1) { set_pte(pte, pfn_pte(pfn, init_prot)); last_map_addr = (pfn << PAGE_SHIFT) + PAGE_SIZE; } else set_pte(pte, pfn_pte(pfn, prot)); } } } if (mapping_iter == 1) { /* * update direct mapping page count only in the first * iteration. */ update_page_count(PG_LEVEL_2M, pages_2m); update_page_count(PG_LEVEL_4K, pages_4k); /* * local global flush tlb, which will flush the previous * mappings present in both small and large page TLB's. */ __flush_tlb_all(); /* * Second iteration will set the actual desired PTE attributes. */ mapping_iter = 2; goto repeat; } return last_map_addr; }
kernel_physical_mapping_init()是建立内核页表的一个关键函数，就是它负责处理物理内存的映射。swapper_pg_dir（来自于/arch/x86/kernel/head_32.s）就是页全局目录的空间了。而页表目录的空间则来自于调用one_page_table_init()申请而得，而one_page_table_init()则是通过调用关系：one_page_table_init()->alloc_low_page()->alloc_low_pages()->memblock_reserve()最后申请而得，同时页全局目录项的熟悉也在这里设置完毕，详细代码这里就不分析了。回到kernel_physical_mapping_init()代码中，该函数里面有个标签repeat，通过mapping_iter结合goto语句的控制，该标签下的代码将会执行两次。第一次执行时，内存映射设置如同head_32.s里面的一样，将页面属性设置为PTE_IDENT_ATTR；第二次执行时，会根据内核的情况设置具体的页面属性，默认是设置为PAGE_KERNEL，但如果经过is_kernel_text判断为内核代码空间，则设置为PAGE_KERNEL_EXEC。最终建立内核页表的同时，完成内存映射。
继续init_memory_mapping()的最后一个关键调用函数add_pfn_range_mapped()：
【file：/arch/x86/mm/init.c】 struct range pfn_mapped[E820_X_MAX]; int nr_pfn_mapped; static void add_pfn_range_mapped(unsigned long start_pfn, unsigned long end_pfn) { nr_pfn_mapped = add_range_with_merge(pfn_mapped, E820_X_MAX, nr_pfn_mapped, start_pfn, end_pfn); nr_pfn_mapped = clean_sort_range(pfn_mapped, E820_X_MAX); max_pfn_mapped = max(max_pfn_mapped, end_pfn); if (start_pfn < (1UL<<(32-PAGE_SHIFT))) max_low_pfn_mapped = max(max_low_pfn_mapped, min(end_pfn, 1UL<<(32-PAGE_SHIFT))); }
该函数主要是将新增内存映射的物理页框范围加入到全局数组pfn_mapped中，其中nr_pfn_mapped用于表示数组中的有效项数量。由此一来，则可以通过内核函数pfn_range_is_mapped来判断指定的物理内存是否被映射，避免了重复映射的情况。
回到init_mem_mapping()继续往下，此时memblock_bottom_up()返回的memblock.bottom_up值仍为false，所以接着走的是else分支，调用memory_map_top_down()，入参为ISA_END_ADDRESS和end。其中end则是通过max_low_pfn << PAGE_SHIFT被设置为内核直接映射的最后页框所对应的地址。
memory_map_top_down()代码实现：
【file：/arch/x86/mm/init.c】 /** * memory_map_top_down - Map [map_start, map_end) top down * @map_start: start address of the target memory range * @map_end: end address of the target memory range * * This function will setup direct mapping for memory range * [map_start, map_end) in top-down. That said, the page tables * will be allocated at the end of the memory, and we map the * memory in top-down. */ static void __init memory_map_top_down(unsigned long map_start, unsigned long map_end) { unsigned long real_end, start, last_start; unsigned long step_size; unsigned long addr; unsigned long mapped_ram_size = 0; unsigned long new_mapped_ram_size; /* xen has big range in reserved near end of ram, skip it at first.*/ addr = memblock_find_in_range(map_start, map_end, PMD_SIZE, PMD_SIZE); real_end = addr + PMD_SIZE; /* step_size need to be small so pgt_buf from BRK could cover it */ step_size = PMD_SIZE; max_pfn_mapped = 0; /* will get exact value next */ min_pfn_mapped = real_end >> PAGE_SHIFT; last_start = start = real_end; /* * We start from the top (end of memory) and go to the bottom. * The memblock_find_in_range() gets us a block of RAM from the * end of RAM in [min_pfn_mapped, max_pfn_mapped) used as new pages * for page table. */ while (last_start > map_start) { if (last_start > step_size) { start = round_down(last_start - 1, step_size); if (start < map_start) start = map_start; } else start = map_start; new_mapped_ram_size = init_range_memory_mapping(start, last_start); last_start = start; min_pfn_mapped = last_start >> PAGE_SHIFT; /* only increase step_size after big range get mapped */ if (new_mapped_ram_size > mapped_ram_size) step_size = get_new_step_size(step_size); mapped_ram_size += new_mapped_ram_size; } if (real_end < map_end) init_range_memory_mapping(real_end, map_end); }
memory_map_top_down()首先使用memblock_find_in_range尝试查找内存，PMD_SIZE大小的内存（4M），确认建立页表的空间足够，然后开始建立页表，其关键函数是init_range_memory_mapping()，该函数的实现：
【file：/arch/x86/mm/init.c】 /* * We need to iterate through the E820 memory map and create direct mappings * for only E820_RAM and E820_KERN_RESERVED regions. We cannot simply * create direct mappings for all pfns from [0 to max_low_pfn) and * [4GB to max_pfn) because of possible memory holes in high addresses * that cannot be marked as UC by fixed/variable range MTRRs. * Depending on the alignment of E820 ranges, this may possibly result * in using smaller size (i.e. 4K instead of 2M or 1G) page tables. * * init_mem_mapping() calls init_range_memory_mapping() with big range. * That range would have hole in the middle or ends, and only ram parts * will be mapped in init_range_memory_mapping(). */ static unsigned long __init init_range_memory_mapping( unsigned long r_start, unsigned long r_end) { unsigned long start_pfn, end_pfn; unsigned long mapped_ram_size = 0; int i; for_each_mem_pfn_range(i, MAX_NUMNODES, &start_pfn, &end_pfn, NULL) { u64 start = clamp_val(PFN_PHYS(start_pfn), r_start, r_end); u64 end = clamp_val(PFN_PHYS(end_pfn), r_start, r_end); if (start >= end) continue; /* * if it is overlapping with brk pgt, we need to * alloc pgt buf from memblock instead. */ can_use_brk_pgt = max(start, (u64)pgt_buf_end<<PAGE_SHIFT) >= min(end, (u64)pgt_buf_top<<PAGE_SHIFT); init_memory_mapping(start, end); mapped_ram_size += end - start; can_use_brk_pgt = true; } return mapped_ram_size; }
可以看到init_range_memory_mapping()调用了前面刚分析的init_memory_mapping()函数，由此可知，它将完成内核直接映射区（低端内存）的页表建立。此外可以注意到pgt_buf_end和pgt_buf_top的使用，在init_memory_mapping()函数调用前，变量can_use_brk_pgt的设置主要是为了避免内存空间重叠，仍然使用页表缓冲区空间。不过这只是64bit系统上才会出现的情况，而32bit系统上面则没有，因为32bit系统的kernel_physical_mapping_init()并不使用alloc_low_page()申请内存，所以不涉及。
至此，内核低端内存页表建立完毕。
