EXTENT(9)

NAME
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 manager

SYNOPSIS
#include <sys/malloc.h>
#include <sys/extent.h>
struct extent *
extent_create(char *name, u_long start, u_long end, void *storage, size_t storagesize, int flags);
void
extent_destroy(struct extent *ex);
int
extent_alloc(struct extent *ex, u_long size, u_long alignment, u_long boundary, int flags, u_long *result);
int
extent_alloc_subregion(struct extent *ex, u_long substart, u_long subend, u_long size, u_long alignment, u_long boundary, u_long flags, u_long *result);
int
extent_alloc1(struct extent *ex, u_long size, u_long alignment, u_long skew, u_long boundary, int flags, u_long *result);
int
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);
int
extent_alloc_region(struct extent *ex, u_long start, u_long size, int flags);
int
extent_free(struct extent *ex, u_long start, u_long size, int flags);
void
extent_print(struct extent *ex);

DESCRIPTION
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 identification in case of an error. If the flag EX_NOCOALESCE is specified, only entire regions may be freed within the extent map, but internal coalescing 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 parameters, regardless of resulting extent fragmentation.

default
    Allocate the smallest region that is capable of holding the request, thus minimizing fragmentation of the extent.

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 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 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 boundary 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 region.

extent_alloc_subregion() is similar to extent_alloc(), but it allows the caller to specify that the allocated region must fall within the subregion from substart to subend inclusive. The other arguments and the return values of extent_alloc_subregion() are otherwise the same as those of extent_alloc().

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 extensions 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 provided.

LOCKING
The extent manager performs all necessary locking (unless EX_EARLY is specified) on the extent map itself, and any other data structures internal to the extent manager. The locks used by the extent manager are simplelocks, and will never sleep ( see lock(9)). This should be taken into account when designing the locking protocol for users of the extent manager.

RETURN VALUES
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 succeeds 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 not specified.

EINTR
    Process received a signal while waiting for the requested region to become available in the extent. Does not apply to extent_free().

EXAMPLES
Here is an example of a (useless) function that uses several of the extent manager routines.

    void
    func()
    {
    	struct extent *foo_ex;
    	u_long region_start;
    	int error;
    	/*
    	 * 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,
    	    EX_NOWAIT, &region_start);
    	if (error)
    		panic("you lose");
    	/*
    	 * Give up the extent.
    	 */
    	extent_destroy(foo_ex);
    }

CODE REFERENCES
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 sys/arch/i386/i386/machdep.c.

SEE ALSO
malloc(9)

HISTORY
The NetBSD extent manager appeared in NetBSD 1.3.

AUTHORS
The NetBSD extent manager was architected and implemented by Jason R. Thorpe <thorpej@NetBSD.org>. Matthias Drochner <drochner@zelux6.zel.kfa-juelich.de> contributed to the initial testing and optimization of the implementation. Chris Demetriou <cgd@NetBSD.org> contributed many architectural suggestions.