cleanup_highmap之后的页表
在x86平台的setup_arch中，会对内核的虚拟机地址空间做一个剪切。具体原因可以看代码的注释。
/*
* The head.S code sets up the kernel high mapping:
*
* from __START_KERNEL_map to __START_KERNEL_map + size (== _end-_text)
*
* phys_base holds the negative offset to the kernel, which is added
* to the compile time generated pmds. This results in invalid pmds up
* to the point where we hit the physaddr 0 mapping.
*
* We limit the mappings to the region from _text to _brk_end. _brk_end
* is rounded up to the 2MB boundary. This catches the invalid pmds as
* well, as they are located before _text:
*/说的有点多，最后的结果比较简单，就是只留下了_text到_brk_end之间的页表映射。
那咱们来打印一下，看看效果呗。
调试补丁
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 14b9dd7..2ffb7f2 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -310,6 +310,25 @@ void __init cleanup_highmap(void)
unsigned long vaddr_end = __START_KERNEL_map + KERNEL_IMAGE_SIZE;
unsigned long end = roundup((unsigned long)_brk_end, PMD_SIZE) - 1;
pmd_t *pmd = level2_kernel_pgt;
+ int i = 0;
+
+ pr_err(": phys_base %lx\n", phys_base );
+ pr_err(": #level2_kernel_pgt %lu\n", KERNEL_IMAGE_SIZE/PMD_SIZE);
+ pr_err(": __START_KERNEL_map %lx\n", __START_KERNEL_map);
+ pr_err(": __START_KERNEL %lx\n", __START_KERNEL);
+ pr_err(": _text %lx\n", (unsigned long)_text);
+ pr_err(": _brk_end %lx\n", (unsigned long)_brk_end);
+ pr_err(": _end %lx\n", (unsigned long)_end);
+ pr_err(": __START_KERNEL_map + KI %lx\n",
+ __START_KERNEL_map + KERNEL_IMAGE_SIZE);
+
+ for (i = 0; i < 512; i++) {
+ if (pmd_none(*(pmd + i)))
+ continue;
+
+ pr_err(": level2_kernel_pgt[%d] = %lx\n",
+ i, pmd_val(*(pmd+i)));
+ }
/*
* Native path, max_pfn_mapped is not set yet.
@@ -325,6 +344,17 @@ void __init cleanup_highmap(void)
if (vaddr < (unsigned long) _text || vaddr > end)
set_pmd(pmd, __pmd(0));
}
+
+ pr_err(": 2nd round\n");
+ pmd = level2_kernel_pgt;
+ for (i = 0; i < 512; i++) {
+ if (pmd_none(*(pmd + i)))
+ continue;
+
+ pr_err(": level2_kernel_pgt[%d] = %lx\n",
+ i, pmd_val(*(pmd+i)));
+ }
+
}
/*稍微有点长，但是功能却比较简单。
打印了几个比较重要的变量
打印了level2_kernel_pgt中的非空项
几个重要的变量
下面打印了内核中几个比较重要的虚拟地址的值，按照大小排序：
注，这个内核我disable了RANDOMIZE_MEMORY。
在没有随机放置内核的情况下 phys_base为0，所以内核的起始地址并没有变化和编译时的一样。
这里要看的是那个KI
这个值就是内核镜像的大小，也就是我们需要做内核地址映射的大小。因为每个PMD映射空间是2MB，所以整个内核地址空间需要映射256个entry。
level2_kernel_pgt的变化
截取调试打印的部分
在cleanup_highmap()之前，level2_kernel_pgt中一共有256项有效项。你看是不是和之前计算的能对应上了。而且每个映射是线性的。
经过cleanup_highmap()之后，level2_kernel_pgt中只有[8..17]项是有效映射了。然后再来仔细分析一下。[8] = 0x1000000 这个值是不是正好是_pa(_text)？而[17] = 0x2200000 又和_brk_end对应上？
长这样
用图来看，应该能够更清楚一些
感觉离真相又近了一步～
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