Linux的初始页表

LOWMEM，是相对于HIGHMEM而言，其实就是内核态直接映射的物理内存空间的大小，一般为896MB。

这里只是粗略地估计了一下，取1GB。

LOWMEM_PAGES代表了LOWMEM区域对应了多少内存页，1GB / 4KB = 256K，即需要256K个页表项来描述内核空间

   1: /* Number of possible pages in the lowmem region */

   2: LOWMEM_PAGES = (((1<<32) - __PAGE_OFFSET) >> PAGE_SHIFT)

 

计算一下，需要多少内存页来存放LOWMEM区域对应的内存页表项，有256个页表，每个页表占1页内存，需要256页，即256 * 4KB = 1MB来存放256个页表。

   1: /* Enough space to fit pagetables for the low memory linear map */

   2: MAPPING_BEYOND_END = PAGE_TABLE_SIZE(LOWMEM_PAGES) << PAGE_SHIFT

其中，PAGE_TABLE_SIZE用来计算需要多少个页表来容纳这么多的页， 256K / 1K = 256，需要256个页表

   1: #if PTRS_PER_PMD > 1

   2: #define PAGE_TABLE_SIZE(pages) (((pages) / PTRS_PER_PMD) + PTRS_PER_PGD)

   3: #else

   4: #define PAGE_TABLE_SIZE(pages) ((pages) / PTRS_PER_PGD)

   5: #endif

下面的计算与上面是相同的， INIT_MAP_SIZE也是1MB

   1: /*

   2:  * Worst-case size of the kernel mapping we need to make:

   3:  * a relocatable kernel can live anywhere in lowmem, so we need to be able

   4:  * to map all of lowmem.

   5:  */

   6: KERNEL_PAGES = LOWMEM_PAGES

   7:  

   8: INIT_MAP_SIZE = PAGE_TABLE_SIZE(KERNEL_PAGES) * PAGE_SIZE

   9: RESERVE_BRK(pagetables, INIT_MAP_SIZE)

RESERVE_BRK的作用是在.brk区域预留1MB大小的空间，并且设置pagetables为该区域的起始位置的标号名称。

   1: /*

   2:  * Reserve space in the brk section.  The name must be unique within

   3:  * the file, and somewhat descriptive.  The size is in bytes.  Must be

   4:  * used at file scope.

   5:  *

   6:  * (This uses a temp function to wrap the asm so we can pass it the

   7:  * size parameter; otherwise we wouldn't be able to.  We can't use a

   8:  * "section" attribute on a normal variable because it always ends up

   9:  * being @progbits, which ends up allocating space in the vmlinux

  10:  * executable.)

  11:  */

  12: #define RESERVE_BRK(name,sz)                        \

  13:     static void __section(.discard.text) __used notrace        \

  14:     __brk_reservation_fn_##name##__(void) {                \

  15:         asm volatile (                        \

  16:             ".pushsection .brk_reservation,\"aw\",@nobits;" \

  17:             ".brk." #name ":"                \

  18:             " 1:.skip %c0;"                    \

  19:             " .size .brk." #name ", . - 1b;"        \

  20:             " .popsection"                    \

  21:             : : "i" (sz));                    \

  22:     }