1.. _highmem:
2
3====================
4High Memory Handling
5====================
6
7By: Peter Zijlstra <a.p.zijlstra@chello.nl>
8
9.. contents:: :local:
10
11What Is High Memory?
12====================
13
14High memory (highmem) is used when the size of physical memory approaches or
15exceeds the maximum size of virtual memory.  At that point it becomes
16impossible for the kernel to keep all of the available physical memory mapped
17at all times.  This means the kernel needs to start using temporary mappings of
18the pieces of physical memory that it wants to access.
19
20The part of (physical) memory not covered by a permanent mapping is what we
21refer to as 'highmem'.  There are various architecture dependent constraints on
22where exactly that border lies.
23
24In the i386 arch, for example, we choose to map the kernel into every process's
25VM space so that we don't have to pay the full TLB invalidation costs for
26kernel entry/exit.  This means the available virtual memory space (4GiB on
27i386) has to be divided between user and kernel space.
28
29The traditional split for architectures using this approach is 3:1, 3GiB for
30userspace and the top 1GiB for kernel space::
31
32		+--------+ 0xffffffff
33		| Kernel |
34		+--------+ 0xc0000000
35		|        |
36		| User   |
37		|        |
38		+--------+ 0x00000000
39
40This means that the kernel can at most map 1GiB of physical memory at any one
41time, but because we need virtual address space for other things - including
42temporary maps to access the rest of the physical memory - the actual direct
43map will typically be less (usually around ~896MiB).
44
45Other architectures that have mm context tagged TLBs can have separate kernel
46and user maps.  Some hardware (like some ARMs), however, have limited virtual
47space when they use mm context tags.
48
49
50Temporary Virtual Mappings
51==========================
52
53The kernel contains several ways of creating temporary mappings. The following
54list shows them in order of preference of use.
55
56* kmap_local_page().  This function is used to require short term mappings.
57  It can be invoked from any context (including interrupts) but the mappings
58  can only be used in the context which acquired them.
59
60  This function should be preferred, where feasible, over all the others.
61
62  These mappings are thread-local and CPU-local, meaning that the mapping
63  can only be accessed from within this thread and the thread is bound to the
64  CPU while the mapping is active. Although preemption is never disabled by
65  this function, the CPU can not be unplugged from the system via
66  CPU-hotplug until the mapping is disposed.
67
68  It's valid to take pagefaults in a local kmap region, unless the context
69  in which the local mapping is acquired does not allow it for other reasons.
70
71  As said, pagefaults and preemption are never disabled. There is no need to
72  disable preemption because, when context switches to a different task, the
73  maps of the outgoing task are saved and those of the incoming one are
74  restored.
75
76  kmap_local_page() always returns a valid virtual address and it is assumed
77  that kunmap_local() will never fail.
78
79  On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the
80  virtual address of the direct mapping. Only real highmem pages are
81  temporarily mapped. Therefore, users may call a plain page_address()
82  for pages which are known to not come from ZONE_HIGHMEM. However, it is
83  always safe to use kmap_local_page() / kunmap_local().
84
85  While it is significantly faster than kmap(), for the higmem case it
86  comes with restrictions about the pointers validity. Contrary to kmap()
87  mappings, the local mappings are only valid in the context of the caller
88  and cannot be handed to other contexts. This implies that users must
89  be absolutely sure to keep the use of the return address local to the
90  thread which mapped it.
91
92  Most code can be designed to use thread local mappings. User should
93  therefore try to design their code to avoid the use of kmap() by mapping
94  pages in the same thread the address will be used and prefer
95  kmap_local_page().
96
97  Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain
98  extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered
99  because the map implementation is stack based. See kmap_local_page() kdocs
100  (included in the "Functions" section) for details on how to manage nested
101  mappings.
102
103* kmap_atomic().  This permits a very short duration mapping of a single
104  page.  Since the mapping is restricted to the CPU that issued it, it
105  performs well, but the issuing task is therefore required to stay on that
106  CPU until it has finished, lest some other task displace its mappings.
107
108  kmap_atomic() may also be used by interrupt contexts, since it does not
109  sleep and the callers too may not sleep until after kunmap_atomic() is
110  called.
111
112  Each call of kmap_atomic() in the kernel creates a non-preemptible section
113  and disable pagefaults. This could be a source of unwanted latency. Therefore
114  users should prefer kmap_local_page() instead of kmap_atomic().
115
116  It is assumed that k[un]map_atomic() won't fail.
117
118* kmap().  This should be used to make short duration mapping of a single
119  page with no restrictions on preemption or migration. It comes with an
120  overhead as mapping space is restricted and protected by a global lock
121  for synchronization. When mapping is no longer needed, the address that
122  the page was mapped to must be released with kunmap().
123
124  Mapping changes must be propagated across all the CPUs. kmap() also
125  requires global TLB invalidation when the kmap's pool wraps and it might
126  block when the mapping space is fully utilized until a slot becomes
127  available. Therefore, kmap() is only callable from preemptible context.
128
129  All the above work is necessary if a mapping must last for a relatively
130  long time but the bulk of high-memory mappings in the kernel are
131  short-lived and only used in one place. This means that the cost of
132  kmap() is mostly wasted in such cases. kmap() was not intended for long
133  term mappings but it has morphed in that direction and its use is
134  strongly discouraged in newer code and the set of the preceding functions
135  should be preferred.
136
137  On 64-bit systems, calls to kmap_local_page(), kmap_atomic() and kmap() have
138  no real work to do because a 64-bit address space is more than sufficient to
139  address all the physical memory whose pages are permanently mapped.
140
141* vmap().  This can be used to make a long duration mapping of multiple
142  physical pages into a contiguous virtual space.  It needs global
143  synchronization to unmap.
144
145
146Cost of Temporary Mappings
147==========================
148
149The cost of creating temporary mappings can be quite high.  The arch has to
150manipulate the kernel's page tables, the data TLB and/or the MMU's registers.
151
152If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping
153simply with a bit of arithmetic that will convert the page struct address into
154a pointer to the page contents rather than juggling mappings about.  In such a
155case, the unmap operation may be a null operation.
156
157If CONFIG_MMU is not set, then there can be no temporary mappings and no
158highmem.  In such a case, the arithmetic approach will also be used.
159
160
161i386 PAE
162========
163
164The i386 arch, under some circumstances, will permit you to stick up to 64GiB
165of RAM into your 32-bit machine.  This has a number of consequences:
166
167* Linux needs a page-frame structure for each page in the system and the
168  pageframes need to live in the permanent mapping, which means:
169
170* you can have 896M/sizeof(struct page) page-frames at most; with struct
171  page being 32-bytes that would end up being something in the order of 112G
172  worth of pages; the kernel, however, needs to store more than just
173  page-frames in that memory...
174
175* PAE makes your page tables larger - which slows the system down as more
176  data has to be accessed to traverse in TLB fills and the like.  One
177  advantage is that PAE has more PTE bits and can provide advanced features
178  like NX and PAT.
179
180The general recommendation is that you don't use more than 8GiB on a 32-bit
181machine - although more might work for you and your workload, you're pretty
182much on your own - don't expect kernel developers to really care much if things
183come apart.
184
185
186Functions
187=========
188
189.. kernel-doc:: include/linux/highmem.h
190.. kernel-doc:: include/linux/highmem-internal.h
191