Tribblix: manual page: vmem.9

VMEM(9) Kernel Concepts VMEM(9)

NAME

vmem - virtual memory allocator

DESCRIPTION

Overview

An address space is divided into a number of logically distinct pieces,
or 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 (kmem_alloc() space), but also for DVMA,
bp_mapin(), /dev/kmem, and even some device mappings.

The kernel address space, therefore, is most accurately described as a
tree of arenas in which each node of the tree imports some subset of
its parent. The virtual memory allocator manages these arenas and
supports their natural hierarchical structure.

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 id_space(9F) instead.

vmem_create() and vmem_destroy() create and destroy vmem arenas. In
order to differentiate between arenas used for addresses and arenas
used for identifiers, the VMC_IDENTIFIER flag is passed to
vmem_create(). This prevents identifier exhaustion from being
diagnosed as general memory failure.

Spans

We represent the integers in an arena as a collection of spans, or
contiguous ranges of integers. For example, the kernel heap consists
of just one span: [kernelheap, ekernelheap). Spans can be added to an
arena in two ways: explicitly, by vmem_add(); or implicitly, by
importing, as described in Imported Memory below.

Segments

Spans are subdivided into segments, each of which is either allocated
or free. A segment, like a span, is a contiguous range of integers.
Each allocated segment [addr, addr + size) represents exactly one
vmem_alloc(size) that returned 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 [a, b) and
[b, c) are both free, we merge them into a single segment [a, c). The
segments within a span are linked together in increasing-address order
so we can easily determine whether coalescing is possible.

Segments never cross span boundaries. When all segments within an
imported span become free, we return the span to its source.

Imported Memory

As mentioned in the overview, some arenas are logical subsets of other
arenas. For example, kmem_va_arena (a virtual address cache that
satisfies most kmem_slab_create() requests) is just a subset of
heap_arena (the kernel heap) that provides caching for the most common
slab sizes. When kmem_va_arena runs out of virtual memory, it imports
more from the heap; we say that heap_arena is the vmem source for
kmem_va_arena. 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.

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) -- or all
of the above. vmem_xalloc() allows the client to specify any or all of
these constraints.

The Vmem Quantum

Every arena has a notion of `quantum', specified at vmem_create() time,
that defines the arena's minimum unit of currency. Most commonly the
quantum is either 1 or PAGESIZE, but any power of 2 is legal. All vmem
allocations are guaranteed to be quantum-aligned.

Relationship to the Kernel Memory Allocator

Every kmem cache has a vmem arena as its slab supplier. The kernel
memory allocator uses vmem_alloc() and vmem_free() to create and
destroy slabs.