VMEM(9)

NAME
vmem - virtual memory allocator

SYNOPSIS
#include <sys/vmem.h>
vmem_t *
vmem_create(const char *name, vmem_addr_t base, vmem_size_t size, vmem_size_t quantum, vmem_addr_t (*allocfn)(vmem_t *, vmem_size_t, vmem_size_t *, vm_flag_t), void (*freefn)(vmem_t *, vmem_addr_t, vmem_size_t), vmem_t *source, vmem_size_t qcache_max, vm_flag_t flags, int ipl);
vmem_addr_t
vmem_add(vmem_t *vm, vmem_addr_t addr, vmem_size_t size, vm_flag_t flags);
vmem_addr_t
vmem_xalloc(vmem_t *vm, vmem_size_t size, vmem_size_t align, vmem_size_t phase, vmem_size_t nocross, vmem_addr_t minaddr, vmem_addr_t maxaddr, vm_flag_t flags);
void
vmem_xfree(vmem_t *vm, vmem_addr_t addr, vmem_size_t size);
vmem_addr_t
vmem_alloc(vmem_t *vm, vmem_size_t size, vm_flag_t flags);
void
vmem_free(vmem_t *vm, vmem_addr_t addr, vmem_size_t size);
void
vmem_destroy(vmem_t *vm);

DESCRIPTION
The vmem is a general purpose resource allocator. Despite its name, it can be used for arbitrary resources other than virtual memory.

vmem_create() creates a new vmem arena.

name
    The string to describe the vmem.

base
    The start address of the initial span. It can be VMEM_ADDR_NULL if no initial span is required.

size
    The size of the initial span.

quantum
    The smallest unit of allocation.

allocfn
    The callback function used to import spans from the backend arena. Set both allocfn and freefn to NULL to disable automatic imports. vmem calls (*allocfn)(source, size, actualsize, flags) to import a span of size at least size. allocfn should accept the same flags as vmem_alloc(). allocfn must return VMEM_ADDR_NULL to indicate failure, or else the starting address of the imported span. If allocfn succeeds, it must write the actual size of the allocation to actualsize. The actual size will always be greater than or equal to the requested size.

freefn
    The callback function used to free spans to the backend arena. freefn may not be NULL unless allocfn is NULL. vmem calls (*freefn)(source, addr, size) to return to source a span of size size, starting at addr, that was previously allocated by allocfn.

source
    The backend arena. source may be NULL. vmem passes source as the first argument of allocfn and freefn.

qcache_max
    The largest size of allocations which can be served by quantum cache. It is merely a hint and can be ignored.

flags
    Either of:

    VM_SLEEP
        Can sleep until enough resources are available.

    VM_NOSLEEP
        Don't sleep. Immediately return NULL if there are not enough resources available.

ipl Interrupt level to be blocked for allocating from vmem.

vmem_add() adds a span of size size starting at addr to the arena. Returns addr on success. flags should be one of:

VM_SLEEP
    Can sleep until enough resources are available.

VM_NOSLEEP
    Don't sleep. Immediately return VMEM_ADDR_NULL if there are not enough resources available.

vmem_xalloc() allocates a resource from the arena.

vm  The arena which we allocate from.

size
    Specify the size of the allocation.

align
    If zero, don't care about the alignment of the allocation. Otherwise, request a resource segment starting at offset phase from an align aligned boundary.

phase
    See the above description of align. If align is zero, phase should be zero. Otherwise, phase should be smaller than align.

nocross
    Request a resource which doesn't cross nocross aligned boundary.

minaddr
    Specify the minimum address which can be allocated, or VMEM_ADDR_MIN if the caller does not care.

maxaddr
    Specify the maximum address which can be allocated, or VMEM_ADDR_MAX if the caller does not care.

flags
    A bitwise OR of an allocation strategy and a sleep flag.

    The allocation strategy is one of:

    VM_BESTFIT
        Prefer space efficiency.

    VM_INSTANTFIT
        Prefer performance.

    The sleep flag should be one of:

    VM_SLEEP
        Can sleep until enough resources are available.

    VM_NOSLEEP
        Don't sleep. Immediately return VMEM_ADDR_NULL if there are not enough resources available.

vmem_xfree() frees resource allocated by vmem_xalloc() to the arena.

vm  The arena which we free to.

addr
    The resource being freed. It must be the one returned by vmem_xalloc(). Notably, it must not be the one from vmem_alloc(). Otherwise, the behaviour is undefined.

size
    The size of the resource being freed. It must be the same as the size argument used for vmem_xalloc().

vmem_alloc() allocates a resource from the arena.

vm  The arena which we allocate from.

size
    Specify the size of the allocation.

flags
    A bitwise OR of an allocation strategy and a sleep flag.

    The allocation strategy is one of:

    VM_BESTFIT
        Prefer space efficiency.

    VM_INSTANTFIT
        Prefer performance.

    The sleep flag should be one of:

    VM_SLEEP
        Can sleep until enough resources are available.

    VM_NOSLEEP
        Don't sleep. Immediately return VMEM_ADDR_NULL if there are not enough resources available.

vmem_free() frees resource allocated by vmem_alloc() to the arena.

vm  The arena which we free to.

addr
    The resource being freed. It must be the one returned by vmem_alloc(). Notably, it must not be the one from vmem_xalloc(). Otherwise, the behaviour is undefined.

size
    The size of the resource being freed. It must be the same as the size argument used for vmem_alloc().

vmem_destroy() destroys a vmem arena.

vm  The vmem arena being destroyed. The caller should ensure that no one will use it anymore.

RETURN VALUES
vmem_create() return a pointer to the newly allocated vmem_t. Otherwise, it returns NULL.

On success, vmem_xalloc() and vmem_alloc() return an allocated vmem_addr_t. Otherwise, VMEM_ADDR_NULL is returned.

CODE REFERENCES
The vmem subsystem is implemented within the file sys/kern/subr_vmem.c.

SEE ALSO
intro(9), kmem(9), memoryallocators(9), uvm(9) Jeff Bonwick, Jonathan Adams, Magazines and Vmem: Extending the Slab Allocator to Many CPUs and Arbitrary Resources, 2001 USENIX Annual Technical Conference, 2001.

AUTHORS
This implementation of vmem was written by YAMAMOTO Takashi.

BUGS
vmem cannot manage a resource that starts at 0, because it reserves the address VMEM_ADDR_NULL (0) for indicating errors.

vmem cannot manage a resource that ends at the maximum vmem_addr_t. This is an implementation limitation.

vmem relies on malloc(9), pool(9), and RUN_ONCE(9), so it cannot be used as early during system bootstrap as extent(9).