man/man9/vmem.9

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.\" The text of this is derived from section 1 of the big theory statement in
.\" usr/src/uts/common/os/vmem.c, the traditional location of this text.  They
.\" should largely be updated in tandem.
.Dd Jan 18, 2017
.Dt VMEM 9
.Os
.Sh NAME
.Nm vmem
.Nd virtual memory allocator
.Sh DESCRIPTION
.Ss Overview
An address space is divided into a number of logically distinct pieces, or
.Em arenas :
text, data, heap, stack, and so on.
Within these
arenas we often subdivide further; for example, we use heap addresses
not only for the kernel heap
.Po
.Fn kmem_alloc
space
.Pc ,
but also for DVMA,
.Fn bp_mapin ,
.Pa /dev/kmem ,
and even some device mappings.
.Pp
The kernel address space, therefore, is most accurately described as
a tree of arenas in which each node of the tree
.Em imports
some subset of its parent.
The virtual memory allocator manages these arenas
and supports their natural hierarchical structure.
.Ss Arenas
An arena is nothing more than a set of integers.
These integers most commonly represent virtual addresses, but in fact they can
represent anything at all.
For example, we could use an arena containing the integers minpid through maxpid
to allocate process IDs.
For uses of this nature, prefer
.Xr id_space 9F
instead.
.Pp
.Fn vmem_create
and
.Fn vmem_destroy
create and destroy vmem arenas.
In order to differentiate between arenas used for addresses and arenas used for
identifiers, the
.Dv VMC_IDENTIFIER
flag is passed to
.Fn vmem_create .
This prevents identifier exhaustion from being diagnosed as general memory
failure.
.Ss Spans
We represent the integers in an arena as a collection of
.Em spans ,
or contiguous ranges of integers.
For example, the kernel heap consists of just one span:
.Li "[kernelheap, ekernelheap)" .
Spans can be added to an arena in two ways: explicitly, by
.Fn vmem_add ;
or implicitly, by importing, as described in
.Sx Imported Memory
below.
.Ss Segments
Spans are subdivided into
.Em segments ,
each of which is either allocated or free.
A segment, like a span, is a contiguous range of integers.
Each allocated segment
.Li "[addr, addr + size)"
represents exactly one
.Li "vmem_alloc(size)"
that returned
.Sy addr .
Free segments represent the space between allocated segments.
If two free segments are adjacent, we coalesce them into one larger segment;
that is, if segments
.Li "[a, b)"
and
.Li "[b, c)"
are both free, we merge them into a single segment
.Li "[a, c)" .
The segments within a span are linked together in increasing\-address
order so we can easily determine whether coalescing is possible.
.Pp
Segments never cross span boundaries.
When all segments within an imported span become free, we return the span to its
source.
.Ss Imported Memory
As mentioned in the overview, some arenas are logical subsets of other arenas.
For example,
.Sy kmem_va_arena
(a virtual address cache
that satisfies most
.Fn kmem_slab_create
requests) is just a subset of
.Sy heap_arena
(the kernel heap) that provides caching for the most common slab sizes.
When
.Sy kmem_va_arena
runs out of virtual memory, it
.Em imports
more from the heap; we say that
.Sy heap_arena
is the
.Em "vmem source"
for
.Sy kmem_va_arena.
.Fn vmem_create
allows you to specify any existing vmem arena as the source for your new arena.
Topologically, since every arena is a child of at most one source, the set of
all arenas forms a collection of trees.
.Ss Constrained Allocations
Some vmem clients are quite picky about the kind of address they want.
For example, the DVMA code may need an address that is at a particular
phase with respect to some alignment (to get good cache coloring), or
that lies within certain limits (the addressable range of a device),
or that doesn't cross some boundary (a DMA counter restriction) \(em
or all of the above.
.Fn vmem_xalloc
allows the client to specify any or all of these constraints.
.Ss The Vmem Quantum
Every arena has a notion of
.Sq quantum ,
specified at
.Fn vmem_create
time, that defines the arena's minimum unit of currency.
Most commonly the quantum is either 1 or
.Dv PAGESIZE ,
but any power of 2 is legal.
All vmem allocations are guaranteed to be quantum\-aligned.
.Ss Relationship to the Kernel Memory Allocator
Every kmem cache has a vmem arena as its slab supplier.
The kernel memory allocator uses
.Fn vmem_alloc
and
.Fn vmem_free
to create and destroy slabs.
.Sh SEE ALSO
.Xr id_space 9F ,
.Xr vmem_add 9F ,
.Xr vmem_alloc 9F ,
.Xr vmem_contains 9F ,
.Xr vmem_create 9F ,
.Xr vmem_walk 9F
.Pp
.Rs
.%A Jeff Bonwick
.%A Jonathan Adams
.%T Magazines and vmem: Extending the Slab Allocator to Many CPUs and Arbitrary Resources.
.%J Proceedings of the 2001 Usenix Conference
.%U http://www.usenix.org/event/usenix01/bonwick.html
.Re