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 the
64  CPU while the mapping is active. Even if the thread is preempted (since
65  preemption is never disabled by the function) the CPU can not be
66  unplugged from the system via 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  kmap_local_page() always returns a valid virtual address and it is assumed
72  that kunmap_local() will never fail.
73
74  Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain
75  extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered
76  because the map implementation is stack based. See kmap_local_page() kdocs
77  (included in the "Functions" section) for details on how to manage nested
78  mappings.
79
80* kmap_atomic().  This permits a very short duration mapping of a single
81  page.  Since the mapping is restricted to the CPU that issued it, it
82  performs well, but the issuing task is therefore required to stay on that
83  CPU until it has finished, lest some other task displace its mappings.
84
85  kmap_atomic() may also be used by interrupt contexts, since it does not
86  sleep and the callers too may not sleep until after kunmap_atomic() is
87  called.
88
89  Each call of kmap_atomic() in the kernel creates a non-preemptible section
90  and disable pagefaults. This could be a source of unwanted latency. Therefore
91  users should prefer kmap_local_page() instead of kmap_atomic().
92
93  It is assumed that k[un]map_atomic() won't fail.
94
95* kmap().  This should be used to make short duration mapping of a single
96  page with no restrictions on preemption or migration. It comes with an
97  overhead as mapping space is restricted and protected by a global lock
98  for synchronization. When mapping is no longer needed, the address that
99  the page was mapped to must be released with kunmap().
100
101  Mapping changes must be propagated across all the CPUs. kmap() also
102  requires global TLB invalidation when the kmap's pool wraps and it might
103  block when the mapping space is fully utilized until a slot becomes
104  available. Therefore, kmap() is only callable from preemptible context.
105
106  All the above work is necessary if a mapping must last for a relatively
107  long time but the bulk of high-memory mappings in the kernel are
108  short-lived and only used in one place. This means that the cost of
109  kmap() is mostly wasted in such cases. kmap() was not intended for long
110  term mappings but it has morphed in that direction and its use is
111  strongly discouraged in newer code and the set of the preceding functions
112  should be preferred.
113
114  On 64-bit systems, calls to kmap_local_page(), kmap_atomic() and kmap() have
115  no real work to do because a 64-bit address space is more than sufficient to
116  address all the physical memory whose pages are permanently mapped.
117
118* vmap().  This can be used to make a long duration mapping of multiple
119  physical pages into a contiguous virtual space.  It needs global
120  synchronization to unmap.
121
122
123Cost of Temporary Mappings
124==========================
125
126The cost of creating temporary mappings can be quite high.  The arch has to
127manipulate the kernel's page tables, the data TLB and/or the MMU's registers.
128
129If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping
130simply with a bit of arithmetic that will convert the page struct address into
131a pointer to the page contents rather than juggling mappings about.  In such a
132case, the unmap operation may be a null operation.
133
134If CONFIG_MMU is not set, then there can be no temporary mappings and no
135highmem.  In such a case, the arithmetic approach will also be used.
136
137
138i386 PAE
139========
140
141The i386 arch, under some circumstances, will permit you to stick up to 64GiB
142of RAM into your 32-bit machine.  This has a number of consequences:
143
144* Linux needs a page-frame structure for each page in the system and the
145  pageframes need to live in the permanent mapping, which means:
146
147* you can have 896M/sizeof(struct page) page-frames at most; with struct
148  page being 32-bytes that would end up being something in the order of 112G
149  worth of pages; the kernel, however, needs to store more than just
150  page-frames in that memory...
151
152* PAE makes your page tables larger - which slows the system down as more
153  data has to be accessed to traverse in TLB fills and the like.  One
154  advantage is that PAE has more PTE bits and can provide advanced features
155  like NX and PAT.
156
157The general recommendation is that you don't use more than 8GiB on a 32-bit
158machine - although more might work for you and your workload, you're pretty
159much on your own - don't expect kernel developers to really care much if things
160come apart.
161
162
163Functions
164=========
165
166.. kernel-doc:: include/linux/highmem.h
167.. kernel-doc:: include/linux/highmem-internal.h
168