EXTENT(9) | Kernel Developer's Manual | 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.

August 28, 2017 | NetBSD 9.1