Kernel page Global Directory (PGD) of page table of Process created in Linux Kernel

KernelPage Global Directory (PGD)of User process created

In earlier versions:

When you fork a process, you must establish the process's own kernel page directory entry (the kernel page directory entry to the user space
Page directory is placed on a contiguous page of the same physical address, so it cannot be shared, but the kernel page table of all processes
Process 0 Sharing)
3G user, a page directory in a map 4M of space (a page table of Contents 1024 Items page tables, each page table corresponds to 1 pages 4K
), namely:
#define Pgdir_shift 22
#define PGDIR_SIZE (1UL << pgdir_shift)
>>> Sys_fork->do_fork->copy_mm->mm_init->pgd_alloc->get_pgd_slow
#if config_x86_pae
。。。。。。。。。。。。。
#else
extern __inline__ pgd_t *get_pgd_slow (void)
{
>>> Allocation page Catalog table (contains 1024 page catalogs), that is, the page directory allocated for a process can be mapped to a space of
1024*4m=4g
pgd_t *PGD = (pgd_t *) __get_free_page (Gfp_kernel);
if (PGD) {
>>> #define USER_PTRS_PER_PGD (task_size/pgdir_size)
>>> task_size is 3G in size, USER_PTRS_PER_PGD is the number of page directory entries corresponding to the user space (
3g/4m=768)
>>> empty page catalog entries for user space
memset (PGD, 0, USER_PTRS_PER_PGD * sizeof (pgd_t));
>>>Copy the No. 768 to 1023 items of the Kernel Page directory table (swapper_pg_dir) to the page directory table of the process
Items from 768 to 1023
memcpy (PGD + USER_PTRS_PER_PGD, Swapper_pg_dir + USER_PTRS_PER_PGD,
(PTRS_PER_PGD-USER_PTRS_PER_PGD) * sizeof (pgd_t));
 }
return PGD;
}
#endif

The newer version:

pgd_t *pgd_alloc (struct mm_struct *mm)  
Pgd_ctor (mm, PGD);   The key code that copies the page catalog entries that point to the kernel space into the project for the newly assigned PGD.
        -
Clone_pgd_range (PGD + kernel_pgd_boundary,    //item points to the KERNEL space
Swapper_pg_dir + kernel_pgd_boundary,  kernel_pgd_ptrs);

Ref.

[1] http://biancheng.dnbcw.info/linux/321897.html

[2] http://m.blog.csdn.net/blog/SunnyBeiKe/6898253