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Man Pages
VMEM(9) FreeBSD Kernel Developer's Manual VMEM(9)

vmem
general purpose resource allocator

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

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. Pass 0 if no initial span is required.
size
The size of the initial span. Pass 0 if no initial span is required.
quantum
The smallest unit of allocation.
qcache_max
The largest size of allocations which can be served by quantum cache. It is merely a hint and can be ignored.
flags
Combination of malloc(9) wait flag and vmem allocation strategy flag:
Prefer allocation performance.
Prefer space efficiency.
vmem_add() adds a span of size size starting at addr to the arena. Returns 0 on success, ENOMEM on failure. flags is malloc(9) wait flag.
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 malloc(9) wait flag. The allocation strategy is one of M_FIRSTFIT and M_BESTFIT.
addrp
On success, if addrp is not NULL, vmem_xalloc() overwrites it with the start address of the allocated span.
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 vmem allocation strategy flag (see above) and a malloc(9) sleep flag.
addrp
On success, if addrp is not NULL, vmem_alloc() overwrites it with the start address of the allocated span.
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.

vmem_create() returns a pointer to the newly allocated vmem_t. Otherwise, it returns NULL.
On success, vmem_xalloc() and vmem_alloc() return 0. Otherwise, ENOMEM is returned.

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

malloc(9)
Jeff Bonwick and Jonathan Adams, Magazines and Vmem: Extending the Slab Allocator to Many CPUs and Arbitrary Resources, 2001 USENIX Annual Technical Conference, 2001.

The vmem allocator was originally implemented in NetBSD. It was introduced in FreeBSD 10.0.

Original implementation of vmem was written by YAMAMOTO Takashi. The FreeBSD port was made by Jeff Roberson.

vmem relies on malloc(9), so it cannot be used as early during system bootstrap.
July 12, 2013 FreeBSD 12.0-RELEASE

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