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EXTENT(9) NetBSD Kernel Developer's Manual EXTENT(9)
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extent, extent_create, extent_destroy, extent_alloc,
extent_alloc_subregion, extent_alloc1, extent_alloc_subregion1,
extent_alloc_region, extent_free, extent_print -- general purpose extent
struct extent *
extent_create(char *name, u_long start, u_long end, void *storage,
size_t storagesize, int flags);
extent_destroy(struct extent *ex);
extent_alloc(struct extent *ex, u_long size, u_long alignment,
u_long boundary, int flags, u_long *result);
extent_alloc_subregion(struct extent *ex, u_long substart, u_long subend,
u_long size, u_long alignment, u_long boundary, u_long flags,
extent_alloc1(struct extent *ex, u_long size, u_long alignment,
u_long skew, u_long boundary, int flags, u_long *result);
extent_alloc_subregion1(struct extent *ex, u_long substart,
u_long subend, u_long size, u_long alignment, u_long skew,
u_long boundary, u_long flags, u_long *result);
extent_alloc_region(struct extent *ex, u_long start, u_long size,
extent_free(struct extent *ex, u_long start, u_long size, int flags);
extent_print(struct extent *ex);
The NetBSD extent manager provides management of areas of memory or other
number spaces (such as I/O ports). An opaque structure called an extent
map keeps track of allocated regions within the number space.
extent_create() creates an extent map managing the space from start to
end inclusive. The extent map will have the name name, used for identi-
fication in case of an error. If the flag EX_NOCOALESCE is specified,
only entire regions may be freed within the extent map, but internal coa-
lescing of regions is disabled so that extent_free() will never have to
allocate a region descriptor and therefore will never fail. The caller
must specify one of the flags EX_NOWAIT or EX_WAITOK, specifying whether
it is okay to wait for memory allocated for extent map overhead.
There are some applications which may want to use an extent map but can't
use malloc() and free(). These applications may provide pre-allocated
storage for all descriptor overhead with the arguments storage and
storagesize. An extent of this type is called a fixed extent. If the
application can safely use malloc() and free(), storage should be NULL.
A fixed extent has a fixed number of region descriptors, so care should
be taken to provide enough storage for them; alternatively, the flag
EX_MALLOCOK may be passed to allocation requests to indicate that a fixed
extent map may be extended using a call to malloc().
If the flag EX_EARLY is specified, no mutex(9) calls are made in the
expectation that mutual exclusion is not available or required.
extent_destroy() destroys the extent map ex, freeing all allocated
regions. If the extent is not a fixed extent, the region and internal
extent descriptors themselves are freed. This function always succeeds.
extent_alloc() allocates a region in extent ex of size size that fits the
provided parameters. There are two distinct allocation policies, which
are selected by the flags argument:
EX_FAST Allocate the first region that fits the provided parame-
ters, regardless of resulting extent fragmentation.
default Allocate the smallest region that is capable of holding
the request, thus minimizing fragmentation of the
The caller must specify if waiting for space in the extent is allowed
using the flag EX_WAITSPACE. If EX_WAITSPACE is not specified, the allo-
cation will fail if the request can not be satisfied without sleeping.
The caller must also specify, using the EX_NOWAIT or EX_WAITOK flags, if
waiting for overhead allocation is allowed. The request will be aligned
to alignment boundaries. Alignment values must be a power of 2. If no
alignment is necessary, the value 1 should be specified. If boundary is
nonzero, the allocated region will not cross any of the numbers which are
a multiple of boundary. If the caller specifies the EX_BOUNDZERO flag,
the boundary lines begin at zero. Otherwise, the boundary lines begin at
the beginning of the extent. The allocated region may begin on a bound-
ary address, but the end of the region will not touch nor cross it. A
boundary argument smaller than the size of the request is invalid. Upon
successful completion, *result will contain the start of the allocated
extent_alloc_subregion() is similar to extent_alloc(), but it allows the
caller to specify that the allocated region must fall within the subre-
gion from substart to subend inclusive. The other arguments and the
return values of extent_alloc_subregion() are otherwise the same as those
extent_alloc_region() allocates the specific region in the extent map ex
beginning at start with the size size. The caller must specify whether
it is okay to wait for the indicated region to be free using the flag
EX_WAITSPACE. If EX_WAITSPACE is not specified, the allocation will fail
if the request can not be satisfied without sleeping. The caller must
also specify, using the EX_NOWAIT or EX_WAITOK flags, if waiting for
overhead allocation is allowed.
The extent_alloc1() and extent_alloc_subregion1() functions are exten-
sions that take one additional argument, skew, that modifies the
requested alignment result in the following way: the value (result -
skew) is aligned to alignment boundaries. skew must be a smaller number
than alignment. Also, a boundary argument smaller than the sum of the
requested skew and the size of the request is invalid.
extent_free() frees a region of size bytes in extent ex starting at
start. If the extent has the EX_NOCOALESCE property, only entire regions
may be freed. If the extent has the EX_NOCOALESCE property and the
caller attempts to free a partial region, behavior is undefined. The
caller must specify one of the flags EX_NOWAIT or EX_WAITOK to specify
whether waiting for memory is okay; these flags have meaning in the event
that allocation of a region descriptor is required during the freeing
process. This situation occurs only when a partial region that begins
and ends in the middle of another region is freed. Behavior is undefined
if invalid arguments are provided.
extent_print() Print out information about extent ex. This function
always succeeds. Behavior is undefined if invalid arguments are pro-
The extent manager performs all necessary locking (unless EX_EARLY is
specified) on the extent map itself, and any other data structures inter-
nal to the extent manager. The locks used by the extent manager are sim-
plelocks, and will never sleep (see lock(9)). This should be taken into
account when designing the locking protocol for users of the extent man-
The behavior of all extent manager functions is undefined if given
invalid arguments. extent_create() returns the extent map on success, or
NULL if it fails to allocate storage for the extent map. It always suc-
ceeds when creating a fixed extent or when given the flag EX_WAITOK.
extent_alloc(), extent_alloc_region(), extent_alloc_subregion(), and
extent_free() return one of the following values:
0 Operation was successful.
ENOMEM If EX_NOWAIT is specified, the extent manager was not
able to allocate a region descriptor for the new region
or to split a region when freeing a partial region.
EAGAIN Requested region is not available and EX_WAITSPACE was
EINTR Process received a signal while waiting for the requested
region to become available in the extent. Does not apply
Here is an example of a (useless) function that uses several of the
extent manager routines.
struct extent *foo_ex;
* Extent "foo" manages a 256k region starting at 0x0 and
* only allows complete regions to be freed so that
* extent_free() never needs to allocate memory.
foo_ex = extent_create("foo", 0x0, 0x3ffff, M_DEVBUF,
NULL, 0, EX_WAITOK | EX_NOCOALESCE);
* Allocate an 8k region, aligned to a 4k boundary, which
* does not cross any of the 3 64k boundaries (at 64k,
* 128k, and 192k) within the extent.
error = extent_alloc(foo_ex, 0x2000, 0x1000, 0x10000,
* Give up the extent.
The extent manager itself is implemented within the file
sys/kern/subr_extent.c. Function prototypes for the framework are
located in sys/sys/extent.h.
The i386 bus management code uses the extent manager for managing I/O
ports and I/O memory. This code is in the file
The NetBSD extent manager appeared in NetBSD 1.3.
The NetBSD extent manager was architected and implemented by Jason R.
Matthias Drochner <email@example.com> contributed to the
initial testing and optimization of the implementation.
Chris Demetriou <cgd@NetBSD.org> contributed many architectural sugges-
NetBSD 10.99 August 28, 2017 NetBSD 10.99