+.. _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
- physical pages into a contiguous virtual space. It needs global
- synchronization to unmap.
+* 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
- 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(). 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
- 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.
+* 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.
- kmap_atomic() may also be used by interrupt contexts, since it is does not
- sleep and the caller may not sleep until after kunmap_atomic() is called.
+ kmap_atomic() may also be used by interrupt contexts, since it is does not
+ sleep and the caller may not sleep until after kunmap_atomic() is called.
- It may be assumed that k[un]map_atomic() won't fail.
+ 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
- pageframes need to live in the permanent mapping, which means:
+* 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
- 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...
+* 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
- 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.
+* 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.
The general recommendation is that you don't use more than 8GiB on a 32-bit
machine - although more might work for you and your workload, you're pretty