linux/Documentation/vm/highmem.txt
<<
>>
Prefs
   1
   2                             ====================
   3                             HIGH MEMORY HANDLING
   4                             ====================
   5
   6By: Peter Zijlstra <a.p.zijlstra@chello.nl>
   7
   8Contents:
   9
  10 (*) What is high memory?
  11
  12 (*) Temporary virtual mappings.
  13
  14 (*) Using kmap_atomic.
  15
  16 (*) Cost of temporary mappings.
  17
  18 (*) i386 PAE.
  19
  20
  21====================
  22WHAT IS HIGH MEMORY?
  23====================
  24
  25High memory (highmem) is used when the size of physical memory approaches or
  26exceeds the maximum size of virtual memory.  At that point it becomes
  27impossible for the kernel to keep all of the available physical memory mapped
  28at all times.  This means the kernel needs to start using temporary mappings of
  29the pieces of physical memory that it wants to access.
  30
  31The part of (physical) memory not covered by a permanent mapping is what we
  32refer to as 'highmem'.  There are various architecture dependent constraints on
  33where exactly that border lies.
  34
  35In the i386 arch, for example, we choose to map the kernel into every process's
  36VM space so that we don't have to pay the full TLB invalidation costs for
  37kernel entry/exit.  This means the available virtual memory space (4GiB on
  38i386) has to be divided between user and kernel space.
  39
  40The traditional split for architectures using this approach is 3:1, 3GiB for
  41userspace and the top 1GiB for kernel space:
  42
  43                +--------+ 0xffffffff
  44                | Kernel |
  45                +--------+ 0xc0000000
  46                |        |
  47                | User   |
  48                |        |
  49                +--------+ 0x00000000
  50
  51This means that the kernel can at most map 1GiB of physical memory at any one
  52time, but because we need virtual address space for other things - including
  53temporary maps to access the rest of the physical memory - the actual direct
  54map will typically be less (usually around ~896MiB).
  55
  56Other architectures that have mm context tagged TLBs can have separate kernel
  57and user maps.  Some hardware (like some ARMs), however, have limited virtual
  58space when they use mm context tags.
  59
  60
  61==========================
  62TEMPORARY VIRTUAL MAPPINGS
  63==========================
  64
  65The kernel contains several ways of creating temporary mappings:
  66
  67 (*) vmap().  This can be used to make a long duration mapping of multiple
  68     physical pages into a contiguous virtual space.  It needs global
  69     synchronization to unmap.
  70
  71 (*) kmap().  This permits a short duration mapping of a single page.  It needs
  72     global synchronization, but is amortized somewhat.  It is also prone to
  73     deadlocks when using in a nested fashion, and so it is not recommended for
  74     new code.
  75
  76 (*) kmap_atomic().  This permits a very short duration mapping of a single
  77     page.  Since the mapping is restricted to the CPU that issued it, it
  78     performs well, but the issuing task is therefore required to stay on that
  79     CPU until it has finished, lest some other task displace its mappings.
  80
  81     kmap_atomic() may also be used by interrupt contexts, since it is does not
  82     sleep and the caller may not sleep until after kunmap_atomic() is called.
  83
  84     It may be assumed that k[un]map_atomic() won't fail.
  85
  86
  87=================
  88USING KMAP_ATOMIC
  89=================
  90
  91When and where to use kmap_atomic() is straightforward.  It is used when code
  92wants to access the contents of a page that might be allocated from high memory
  93(see __GFP_HIGHMEM), for example a page in the pagecache.  The API has two
  94functions, and they can be used in a manner similar to the following:
  95
  96        /* Find the page of interest. */
  97        struct page *page = find_get_page(mapping, offset);
  98
  99        /* Gain access to the contents of that page. */
 100        void *vaddr = kmap_atomic(page);
 101
 102        /* Do something to the contents of that page. */
 103        memset(vaddr, 0, PAGE_SIZE);
 104
 105        /* Unmap that page. */
 106        kunmap_atomic(vaddr);
 107
 108Note that the kunmap_atomic() call takes the result of the kmap_atomic() call
 109not the argument.
 110
 111If you need to map two pages because you want to copy from one page to
 112another you need to keep the kmap_atomic calls strictly nested, like:
 113
 114        vaddr1 = kmap_atomic(page1);
 115        vaddr2 = kmap_atomic(page2);
 116
 117        memcpy(vaddr1, vaddr2, PAGE_SIZE);
 118
 119        kunmap_atomic(vaddr2);
 120        kunmap_atomic(vaddr1);
 121
 122
 123==========================
 124COST OF TEMPORARY MAPPINGS
 125==========================
 126
 127The cost of creating temporary mappings can be quite high.  The arch has to
 128manipulate the kernel's page tables, the data TLB and/or the MMU's registers.
 129
 130If CONFIG_HIGHMEM is not set, then the kernel will try and create a mapping
 131simply with a bit of arithmetic that will convert the page struct address into
 132a pointer to the page contents rather than juggling mappings about.  In such a
 133case, the unmap operation may be a null operation.
 134
 135If CONFIG_MMU is not set, then there can be no temporary mappings and no
 136highmem.  In such a case, the arithmetic approach will also be used.
 137
 138
 139========
 140i386 PAE
 141========
 142
 143The i386 arch, under some circumstances, will permit you to stick up to 64GiB
 144of RAM into your 32-bit machine.  This has a number of consequences:
 145
 146 (*) Linux needs a page-frame structure for each page in the system and the
 147     pageframes need to live in the permanent mapping, which means:
 148
 149 (*) you can have 896M/sizeof(struct page) page-frames at most; with struct
 150     page being 32-bytes that would end up being something in the order of 112G
 151     worth of pages; the kernel, however, needs to store more than just
 152     page-frames in that memory...
 153
 154 (*) PAE makes your page tables larger - which slows the system down as more
 155     data has to be accessed to traverse in TLB fills and the like.  One
 156     advantage is that PAE has more PTE bits and can provide advanced features
 157     like NX and PAT.
 158
 159The general recommendation is that you don't use more than 8GiB on a 32-bit
 160machine - although more might work for you and your workload, you're pretty
 161much on your own - don't expect kernel developers to really care much if things
 162come apart.
 163
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.