docs/vm: highmem.txt: convert to ReST format

.. _highmem:

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

			     HIGH MEMORY HANDLING

High Memory Handling

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

By: Peter Zijlstra <a.p.zijlstra@chello.nl>

Contents:

 (*) What is high memory?

 (*) Temporary virtual mappings.

 (*) Using kmap_atomic.

 (*) Cost of temporary mappings.

 (*) i386 PAE.

.. contents:: :local:

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

WHAT IS HIGH MEMORY?

What Is High Memory?

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

High memory (highmem) is used when the size of physical memory approaches or

i386) has to be divided between user and kernel space.

The traditional split for architectures using this approach is 3:1, 3GiB for

userspace and the top 1GiB for kernel space:

userspace and the top 1GiB for kernel space::

		+--------+ 0xffffffff

		| Kernel |

space when they use mm context tags.

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

TEMPORARY VIRTUAL MAPPINGS

Temporary Virtual Mappings

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

The kernel contains several ways of creating temporary mappings:

 (*) vmap().  This can be used to make a long duration mapping of multiple

* vmap().  This can be used to make a long duration mapping of multiple

  physical pages into a contiguous virtual space.  It needs global

  synchronization to unmap.

 (*) kmap().  This permits a short duration mapping of a single page.  It needs

* kmap().  This permits a short duration mapping of a single page.  It needs

  global synchronization, but is amortized somewhat.  It is also prone to

  deadlocks when using in a nested fashion, and so it is not recommended for

  new code.

 (*) kmap_atomic().  This permits a very short duration mapping of a single

* kmap_atomic().  This permits a very short duration mapping of a single

  page.  Since the mapping is restricted to the CPU that issued it, it

  performs well, but the issuing task is therefore required to stay on that

  CPU until it has finished, lest some other task displace its mappings.

  It may be assumed that k[un]map_atomic() won't fail.

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

USING KMAP_ATOMIC

Using kmap_atomic

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

When and where to use kmap_atomic() is straightforward.  It is used when code

wants to access the contents of a page that might be allocated from high memory

(see __GFP_HIGHMEM), for example a page in the pagecache.  The API has two

functions, and they can be used in a manner similar to the following:

functions, and they can be used in a manner similar to the following::

	/* Find the page of interest. */

	struct page *page = find_get_page(mapping, offset);

not the argument.

If you need to map two pages because you want to copy from one page to

another you need to keep the kmap_atomic calls strictly nested, like:

another you need to keep the kmap_atomic calls strictly nested, like::

	vaddr1 = kmap_atomic(page1);

	vaddr2 = kmap_atomic(page2);

	kunmap_atomic(vaddr1);

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

COST OF TEMPORARY MAPPINGS

Cost of Temporary Mappings

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

The cost of creating temporary mappings can be quite high.  The arch has to

highmem.  In such a case, the arithmetic approach will also be used.

========

i386 PAE

========

The i386 arch, under some circumstances, will permit you to stick up to 64GiB

of RAM into your 32-bit machine.  This has a number of consequences:

 (*) Linux needs a page-frame structure for each page in the system and the

* Linux needs a page-frame structure for each page in the system and the

  pageframes need to live in the permanent mapping, which means:

 (*) you can have 896M/sizeof(struct page) page-frames at most; with struct

* you can have 896M/sizeof(struct page) page-frames at most; with struct

  page being 32-bytes that would end up being something in the order of 112G

  worth of pages; the kernel, however, needs to store more than just

  page-frames in that memory...

 (*) PAE makes your page tables larger - which slows the system down as more

* PAE makes your page tables larger - which slows the system down as more

  data has to be accessed to traverse in TLB fills and the like.  One

  advantage is that PAE has more PTE bits and can provide advanced features

  like NX and PAT.