VMEM(9) NetBSD Kernel Developer's Manual 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, int (*allocfn)(void *, vmem_size_t, vm_flag_t, vmem_addr_t *), void (*freefn)(void *, vmem_addr_t, vmem_size_t), void *arg, vmem_size_t qcache_max, vm_flag_t flags, int ipl); vmem_t * vmem_xcreate(const char *name, vmem_addr_t base, vmem_size_t size, vmem_size_t quantum, int (*allocfn)(void *, vmem_size_t, vmem_size_t *, vm_flag_t, vmem_addr_t *), void (*freefn)(void *, vmem_addr_t, vmem_size_t), void *arg, vmem_size_t qcache_max, vm_flag_t flags, int ipl); int vmem_add(vmem_t *vm, vmem_addr_t addr, vmem_size_t size, vm_flag_t flags); int 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, vmem_addr_t *addrp); void vmem_xfree(vmem_t *vm, vmem_addr_t addr, vmem_size_t size); void vmem_xfreeall(vmem_t *vm); int vmem_alloc(vmem_t *vm, vmem_size_t size, vm_flag_t 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);
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. 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. 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)(arg, size, flags, &addrp) to import a span of size at least size. allocfn must accept the same flags as vmem_alloc(). allocfn must return ENOMEM to indicate failure, or 0 on success. If allocfn succeeds, it must write the starting address of the imported span to addrp. freefn The callback function used to free spans to the backend arena. freefn may be NULL even if allocfn is not NULL. vmem calls (*freefn)(arg, addr, size) to return to arg a span of size size, starting at addr, that was previ- ously allocated by allocfn. arg The backend arena. arg may be NULL. vmem passes arg 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 If the allocation cannot be satisfied imme- diately, 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_xcreate() 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. 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)(arg, size, &actualsize, flags, &addrp) to import a span of size at least size. allocfn must accept the same flags as vmem_alloc(). allocfn must return ENOMEM to indi- cate failure, or 0 on success. If allocfn succeeds, it must write the actual size of the allocation to actualsize and the starting address of the imported span to addrp. 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 be NULL even if allocfn is not NULL. vmem calls (*freefn)(arg, addr, size) to return to arg a span of size size, starting at addr, that was previ- ously allocated by allocfn. arg The backend arena. arg may be NULL. vmem passes arg 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 If the allocation cannot be satisfied imme- diately, 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 0 on success, ENOMEM on failure. flags Either of: VM_SLEEP If the allocation cannot be satisfied immedi- ately, sleep until enough resources are avail- able. VM_NOSLEEP Don't sleep. Immediately return ENOMEM 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 must be zero. Otherwise, phase must 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 must be one of: VM_BESTFIT Prefer space efficiency. VM_INSTANTFIT Prefer performance. The sleep flag must be one of: VM_SLEEP If the allocation cannot be satisfied immedi- ately, sleep until enough resources are avail- able. VM_NOSLEEP Don't sleep. Immediately return ENOMEM if there are not enough resources available. 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 have been allocated via vmem_xalloc(). Notably, it must not have been allocated via 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_xfreeall() frees all resources that have been allocated by vmem_xalloc() to the arena. vm The arena which we free to. Note that this function is may not be used on arenas where resources have been allocated using vmem_alloc() or arenas that have a quantum cache (i.e. were created with a non-zero qcache_max). 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 must be one of: VM_BESTFIT Prefer space efficiency. VM_INSTANTFIT Prefer performance. The sleep flag must be one of: VM_SLEEP If the allocation cannot be satisfied immedi- ately, sleep until enough resources are avail- able. VM_NOSLEEP Don't sleep. Immediately return ENOMEM if there are not enough resources available. 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 have been allocated via vmem_alloc(). Notably, it must not have been allocated via 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 must ensure that no one will use it anymore.
RETURN VALUES
vmem_create() and vmem_xcreate() return a pointer to the newly allocated vmem_t on success, or NULL if VM_NOSLEEP was specified and memory could not be allocated immediately. vmem_add() returns 0 on success, or ENOMEM if VM_NOSLEEP was specified and memory could not be allocated immediately to record the region. vmem_alloc() and vmem_xalloc() return 0 on success, or ENOMEM if either: - VM_NOSLEEP was specified and a matching region could not be allocated immediately; or - non-default align, phase, or nocross parameters were specified, and a matching region could not be allocated without calling the backing allocfn passed to vmem_create().
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 and 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 relies on malloc(9), pool(9), and RUN_ONCE(9), so it cannot be used as early during system bootstrap as extent(9). vmem has no way to pass align, phase, nocross, minaddr, or maxaddr con- straints into the backing allocator allocfn, so even if VM_SLEEP is spec- ified, vmem_alloc() and vmem_xalloc() may spuriously fail immediately with align, phase, or nocross, or sleep forever with minaddr or maxaddr. NetBSD 10.1 June 15, 2020 NetBSD 10.1
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