7    Overview
   8      An address space is divided into a number of logically distinct pieces,
   9      or arenas: text, data, heap, stack, and so on.  Within these arenas we
  10      often subdivide further; for example, we use heap addresses not only for
  11      the kernel heap (kmem_alloc() space), but also for DVMA, bp_mapin(),
  12      /dev/kmem, and even some device mappings.
  13 
  14      The kernel address space, therefore, is most accurately described as a
  15      tree of arenas in which each node of the tree imports some subset of its
  16      parent.  The virtual memory allocator manages these arenas and supports
  17      their natural hierarchical structure.
  18 
  19    Arenas
  20      An arena is nothing more than a set of integers.  These integers most
  21      commonly represent virtual addresses, but in fact they can represent
  22      anything at all.  For example, we could use an arena containing the
  23      integers minpid through maxpid to allocate process IDs.  For uses of this
  24      nature, prefer id_space(9F) instead.
  25 
  26      vmem_create() and vmem_destroy() create and destroy vmem arenas.  In
  27      order to differentiate between arenas used for adresses and arenas used
  28      for identifiers, the VMC_IDENTIFIER flag is passed to vmem_create().
  29      This prevents identifier exhaustion from being diagnosed as general
  30      memory failure.
  31 
  32    Spans
  33      We represent the integers in an arena as a collection of spans, or
  34      contiguous ranges of integers.  For example, the kernel heap consists of
  35      just one span: [kernelheap, ekernelheap).  Spans can be added to an arena
  36      in two ways: explicitly, by vmem_add(), or implicitly, by importing, as
  37      described in Imported Memory below.
  38 
  39    Segments
  40      Spans are subdivided into segments, each of which is either allocated or
  41      free.  A segment, like a span, is a contiguous range of integers.  Each
  42      allocated segment [addr, addr + size) represents exactly one
  43      vmem_alloc(size) that returned addr.  Free segments represent the space
  44      between allocated segments.  If two free segments are adjacent, we
  45      coalesce them into one larger segment; that is, if segments [a, b) and
  46      [b, c) are both free, we merge them into a single segment [a, c).  The
  47      segments within a span are linked together in increasing-address order so
  48      we can easily determine whether coalescing is possible.
  49 
  50      Segments never cross span boundaries.  When all segments within an
  51      imported span become free, we return the span to its source.
  52 
  53    Imported Memory
  54      As mentioned in the overview, some arenas are logical subsets of other
  55      arenas.  For example, kmem_va_arena (a virtual address cache that
  56      satisfies most kmem_slab_create() requests) is just a subset of

   7    Overview
   8      An address space is divided into a number of logically distinct pieces,
   9      or arenas: text, data, heap, stack, and so on.  Within these arenas we
  10      often subdivide further; for example, we use heap addresses not only for
  11      the kernel heap (kmem_alloc() space), but also for DVMA, bp_mapin(),
  12      /dev/kmem, and even some device mappings.
  13 
  14      The kernel address space, therefore, is most accurately described as a
  15      tree of arenas in which each node of the tree imports some subset of its
  16      parent.  The virtual memory allocator manages these arenas and supports
  17      their natural hierarchical structure.
  18 
  19    Arenas
  20      An arena is nothing more than a set of integers.  These integers most
  21      commonly represent virtual addresses, but in fact they can represent
  22      anything at all.  For example, we could use an arena containing the
  23      integers minpid through maxpid to allocate process IDs.  For uses of this
  24      nature, prefer id_space(9F) instead.
  25 
  26      vmem_create() and vmem_destroy() create and destroy vmem arenas.  In
  27      order to differentiate between arenas used for addresses and arenas used
  28      for identifiers, the VMC_IDENTIFIER flag is passed to vmem_create().
  29      This prevents identifier exhaustion from being diagnosed as general
  30      memory failure.
  31 
  32    Spans
  33      We represent the integers in an arena as a collection of spans, or
  34      contiguous ranges of integers.  For example, the kernel heap consists of
  35      just one span: [kernelheap, ekernelheap).  Spans can be added to an arena
  36      in two ways: explicitly, by vmem_add(); or implicitly, by importing, as
  37      described in Imported Memory below.
  38 
  39    Segments
  40      Spans are subdivided into segments, each of which is either allocated or
  41      free.  A segment, like a span, is a contiguous range of integers.  Each
  42      allocated segment [addr, addr + size) represents exactly one
  43      vmem_alloc(size) that returned addr.  Free segments represent the space
  44      between allocated segments.  If two free segments are adjacent, we
  45      coalesce them into one larger segment; that is, if segments [a, b) and
  46      [b, c) are both free, we merge them into a single segment [a, c).  The
  47      segments within a span are linked together in increasing-address order so
  48      we can easily determine whether coalescing is possible.
  49 
  50      Segments never cross span boundaries.  When all segments within an
  51      imported span become free, we return the span to its source.
  52 
  53    Imported Memory
  54      As mentioned in the overview, some arenas are logical subsets of other
  55      arenas.  For example, kmem_va_arena (a virtual address cache that
  56      satisfies most kmem_slab_create() requests) is just a subset of