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.\" $FreeBSD: src/share/man/man9/zone.9,v 1.9.2.4 2002/05/02 20:01:29 asmodai Exp $
.\" $DragonFly: src/share/man/man9/zone.9,v 1.3 2003/10/15 15:36:44 hmp Exp $
.\"
.Dd January 27, 2001
.Dt ZONE 9
.Os
.Sh NAME
.Nm zbootinit ,
.Nm zinitna ,
.Nm zinit ,
.Nm zalloc ,
.Nm zfree ,
.Nd zone allocator
.Sh SYNOPSIS
.In sys/param.h
.In sys/queue.h
.In vm/vm_zone.h
.Ft void
.Fn zbootinit "vm_zone_t z" "char *name" "int size" "void *item" "int nitems"
.Ft int
.Fn zinitna "vm_zone_t z" "struct vm_object *obj" "char *name" "int size" "int nentries" "int flags" "int zalloc"
.Ft vm_zone_t
.Fn zinit "char *name" "int size" "int nentries" "int flags" "int zalloc"
.Ft void *
.Fn zalloc "vm_zone_t z"
.Ft void
.Fn zfree "vm_zone_t z" "void *item"
.Sh DESCRIPTION
The zone allocator provides an efficient interface for managing
dynamically-sized collections of items of similar size.
The zone allocator can work with preallocated zones as well as with
runtime-allocated ones, and is therefore available much earlier in the
boot process than other memory management routines.
.Pp
A zone is an extensible collection of items of identical size.
The zone allocator keeps track of which items are in use and which
are not, and provides functions for allocating items from the zone and
for releasing them back (which makes them available for later use).
.Pp
The zone allocator stores state information inside the items proper
while they are not allocated,
so structures that will be managed by the zone allocator
and wish to use the type stable property of zones by leaving some fields
pre-filled between allocations, must reserve
two pointers at the very beginning for internal use by the zone
allocator, as follows:
.Bd -literal
struct my_item {
        struct my_item  *z_rsvd1;
        struct my_item  *z_rsvd2;
        /* rest of structure */
};
.Ed
.Pp
Alternatively they should assume those entries corrupted
after each allocation.
After the first allocation of an item,
it will have been cleared to zeroes, however subsequent allocations
will retain the contents as of the last free, with the exception of the
fields mentioned above.
.Pp
Zones are created in one of two fashions, depending how far along the
boot process is.
.Pp
If the VM system is fully initialized, a dynamically allocated zone can
be created using
.Fn zinit .
The
.Fa name
argument should be a pointer to a short, descriptive name for the
zone; it is used for statistics and debugging purposes.
The
.Fa size
and
.Fa nentries
are the size of the items held by the zone and the initial size (in
items) of the zone, respectively.
The
.Fa flags
argument should be set to
.Dv ZONE_INTERRUPT
if there is a chance that items may be allocated from the zone in
interrupt context; note that in this case, the zone will never grow
larger than
.Fa nentries
items.
In all other cases,
.Fa flags
should be set to 0.
The final argument,
.Fa zalloc ,
indicates the number of VM pages by which the zone should grow every
time it fills up.
.Pp
If the VM system is not yet fully initialized, the zone allocator
cannot dynamically allocate VM pages from which to dole out items, so
the caller needs to provide a static pool of items.
In this case, the initialization is done in two stages: first,
.Fn zbootinit
is called before first use of the zone; later, when the VM system is
up, the initialization of the zone is completed by calling
.Fn zinitna .
.Pp
The first argument to
.Fn zbootinit
is a pointer to a static
.Vt "struct vm_zone"
to initialize.
The second and third are the name of the zone and the size of the
items it will hold.
The fourth argument is a pointer to a static array of items from which
the zone allocator will draw until the zone is fully initialized.
The
.Fa nitems
argument is the number of items in the array.
.Pp
The arguments to
.Fa zinitna
are the same as for
.Fa zinit ,
with the addition of a pointer to the zone to initialize, and a
pointer to a
.Vt "struct vm_object"
from which to allocate pages in the
.Dv ZONE_INTERRUPT
case.
.Pp
To allocate an item from a zone, simply call
.Fn zalloc
with a pointer to that zone; it will return a pointer to an item, or
.Dv NULL
in the rare case where all items in the zone are in use and the
allocator is unable to grow the zone.
The
.Fn zalloc
and
.Fn zfree
functions are interrupt and SMP safe.
.Pp
Items are released back to the zone from which they were allocated by
calling
.Fn zfree
with a pointer to the zone and a pointer to the item.
.Sh RETURN VALUES
The
.Fn zinitna
function returns 1 on success and 0 on failure; the only failure case
is inability to preallocate address space for an interrupt-safe zone.
.Pp
The
.Fn zinit
function returns a pointer to a fully initialized
.Vt "struct vm_zone" ,
or
.Dv NULL
if it was unable to
.Fn malloc
a
.Vt "struct vm_zone"
or the
.Dv ZONE_INTERRUPT
flag was specified and
.Fn zinitna
failed to preallocate address space.
.Pp
The
.Fn zalloc
function returns a pointer to an item, or
.Dv NULL
if the zone ran out of unused items and the allocator was unable to
enlarge it.
.Sh SEE ALSO
.Xr malloc 9
.Sh HISTORY
The zone allocator first appeared in
.Fx 3.0 .
.Sh AUTHORS
.An -nosplit
The zone allocator was written by
.An John S. Dyson .
.Pp
This manual page was written by
.An Dag-Erling Co\(:idan Sm\(/orgrav Aq des@FreeBSD.org .