docs/vm: split_page_table_lock: convert to ReST format

.. _split_page_table_lock:

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Split page table lock

=====================

tables. Access to higher level tables protected by mm->page_table_lock.

There are helpers to lock/unlock a table and other accessor functions:

 - pte_offset_map_lock()

	maps pte and takes PTE table lock, returns pointer to the taken

	lock;

tables and the architecture supports it (see below).

Hugetlb and split page table lock

---------------------------------

=================================

Hugetlb can support several page sizes. We use split lock only for PMD

level, but not for PUD.

Hugetlb-specific helpers:

 - huge_pte_lock()

	takes pmd split lock for PMD_SIZE page, mm->page_table_lock

	otherwise;

	returns pointer to table lock;

Support of split page table lock by an architecture

---------------------------------------------------

===================================================

There's no need in special enabling of PTE split page table lock:

everything required is done by pgtable_page_ctor() and pgtable_page_dtor(),

be handled properly.

page->ptl

---------

=========

page->ptl is used to access split page table lock, where 'page' is struct

page of page containing the table. It shares storage with page->private

To avoid increasing size of struct page and have best performance, we use a

trick:

 - if spinlock_t fits into long, we use page->ptr as spinlock, so we

   can avoid indirect access and save a cache line.

 - if size of spinlock_t is bigger then size of long, we use page->ptl as