As for vmalloc the kernel modifies the entries of the master kernel Page Global
Directory and its child Page Tables (see Section 2.5.5), but it leaves unchanged the entries of the process Page Tables mapping the fourth gigabyte. This is fine because the kernel never reclaims Page Middle Directories and Page Tables rooted at the master kernel Page Global Directory.
For instance, suppose that a process in Kernel Mode accessed a noncontiguous memory area that later got released. The process's Page Global Directory entries are equal to the corresponding entries of the master kernel Page Global Directory, thanks to the mechanism explained in Section 8.4; they point to the same Page Middle Directories and Page Tables. The vmfree_area_pages( ) function clears only the entries of the Page Tables (without reclaiming the Page Tables themselves). Further accesses of the process to the released noncontiguous memory area will trigger Page Faults because of the null Page Table entries. However, the handler will consider such accesses a bug because the master kernel Page Tables do not include valid entries.
Continue reading here: Process Address Space
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