Docs/mm: fix typos and grammar in page_tables.rst

the last page of physical memory the external address bus of the CPU can

address.

With a page granularity of 4KB and a address range of 32 bits, pfn 0 is at

With a page granularity of 4KB and an address range of 32 bits, pfn 0 is at

address 0x00000000, pfn 1 is at address 0x00001000, pfn 2 is at 0x00002000

and so on until we reach pfn 0xfffff at 0xfffff000. With 16KB pages pfs are

and so on until we reach pfn 0xfffff at 0xfffff000. With 16KB pages pfns are

at 0x00004000, 0x00008000 ... 0xffffc000 and pfn goes from 0 to 0x3ffff.

As you can see, with 4KB pages the page base address uses bits 12-31 of the

Over time a deeper hierarchy has been developed in response to increasing memory

sizes. When Linux was created, 4KB pages and a single page table called

`swapper_pg_dir` with 1024 entries was used, covering 4MB which coincided with

the fact that Torvald's first computer had 4MB of physical memory. Entries in

this single table were referred to as *PTE*:s - page table entries.

the fact that Torvalds's first computer had 4MB of physical memory. Entries in

this single table were referred to as *PTEs* - page table entries.

The software page table hierarchy reflects the fact that page table hardware has

become hierarchical and that in turn is done to save page table memory and

Additionally, page faults may be also caused by code bugs or by maliciously

crafted addresses that the CPU is instructed to access. A thread of a process

could use instructions to address (non-shared) memory which does not belong to

its own address space, or could try to execute an instruction that want to write

its own address space, or could try to execute an instruction that wants to write

to a read-only location.

If the above-mentioned conditions happen in user-space, the kernel sends a

add that the page faults handler can be disabled and enabled respectively with

`pagefault_disable()` and `pagefault_enable()`.

Several code path make use of the latter two functions because they need to

Several code paths make use of the latter two functions because they need to

disable traps into the page faults handler, mostly to prevent deadlocks.