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critical section. uma_zalloc_arg()/uma_zalloc_free() may acquire a sleepable lock on the zone. The malloc() family of functions may call uma_zalloc_arg() or uma_zalloc_free(). The malloc(9) man page currently claims that free() will never sleep. It also implies that the malloc() family of functions will not sleep when called with M_NOWAIT. However, it is more correct to say that these functions will not sleep indefinitely. Indeed, they may acquire a sleepable lock. However, a developer may overlook this restriction because the WITNESS check that catches attempts to call the malloc() family of functions within a critical section is inconsistenly applied. This change clarifies the language of the malloc(9) man page to clarify the restriction against calling the malloc() family of functions while in a critical section or holding a spin lock. It also adds KASSERTs at appropriate points to make the enforcement of this restriction more consistent. PR: 204633 Differential Revision: https://reviews.freebsd.org/D4197 Reviewed by: markj Approved by: gnn (mentor) Sponsored by: Juniper Networks
282 lines
6.7 KiB
Groff
282 lines
6.7 KiB
Groff
.\"
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.\" Copyright (c) 1996 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Paul Kranenburg.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE
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.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
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.\" $FreeBSD$
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.\"
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.Dd November 19, 2015
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.Dt MALLOC 9
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.Os
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.Sh NAME
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.Nm malloc ,
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.Nm free ,
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.Nm realloc ,
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.Nm reallocf ,
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.Nm MALLOC_DEFINE ,
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.Nm MALLOC_DECLARE
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.Nd kernel memory management routines
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/malloc.h
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.Ft void *
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.Fn malloc "unsigned long size" "struct malloc_type *type" "int flags"
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.Ft void
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.Fn free "void *addr" "struct malloc_type *type"
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.Ft void *
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.Fn realloc "void *addr" "unsigned long size" "struct malloc_type *type" "int flags"
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.Ft void *
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.Fn reallocf "void *addr" "unsigned long size" "struct malloc_type *type" "int flags"
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.Fn MALLOC_DECLARE type
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.In sys/param.h
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.In sys/malloc.h
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.In sys/kernel.h
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.Fn MALLOC_DEFINE type shortdesc longdesc
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.Sh DESCRIPTION
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The
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.Fn malloc
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function allocates uninitialized memory in kernel address space for an
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object whose size is specified by
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.Fa size .
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.Pp
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The
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.Fn free
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function releases memory at address
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.Fa addr
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that was previously allocated by
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.Fn malloc
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for re-use.
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The memory is not zeroed.
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If
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.Fa addr
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is
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.Dv NULL ,
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then
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.Fn free
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does nothing.
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.Pp
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The
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.Fn realloc
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function changes the size of the previously allocated memory referenced by
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.Fa addr
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to
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.Fa size
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bytes.
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The contents of the memory are unchanged up to the lesser of the new and
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old sizes.
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Note that the returned value may differ from
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.Fa addr .
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If the requested memory cannot be allocated,
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.Dv NULL
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is returned and the memory referenced by
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.Fa addr
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is valid and unchanged.
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If
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.Fa addr
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is
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.Dv NULL ,
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the
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.Fn realloc
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function behaves identically to
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.Fn malloc
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for the specified size.
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.Pp
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The
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.Fn reallocf
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function is identical to
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.Fn realloc
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except that it
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will free the passed pointer when the requested memory cannot be allocated.
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.Pp
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Unlike its standard C library counterpart
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.Pq Xr malloc 3 ,
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the kernel version takes two more arguments.
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The
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.Fa flags
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argument further qualifies
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.Fn malloc Ns 's
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operational characteristics as follows:
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.Bl -tag -width indent
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.It Dv M_ZERO
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Causes the allocated memory to be set to all zeros.
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.It Dv M_NODUMP
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For allocations greater than page size, causes the allocated
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memory to be excluded from kernel core dumps.
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.It Dv M_NOWAIT
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Causes
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.Fn malloc ,
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.Fn realloc ,
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and
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.Fn reallocf
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to return
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.Dv NULL
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if the request cannot be immediately fulfilled due to resource shortage.
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Note that
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.Dv M_NOWAIT
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is required when running in an interrupt context.
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.It Dv M_WAITOK
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Indicates that it is OK to wait for resources.
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If the request cannot be immediately fulfilled, the current process is put
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to sleep to wait for resources to be released by other processes.
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The
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.Fn malloc ,
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.Fn realloc ,
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and
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.Fn reallocf
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functions cannot return
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.Dv NULL
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if
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.Dv M_WAITOK
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is specified.
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.It Dv M_USE_RESERVE
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Indicates that the system can use its reserve of memory to satisfy the
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request.
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This option should only be used in combination with
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.Dv M_NOWAIT
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when an allocation failure cannot be tolerated by the caller without
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catastrophic effects on the system.
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.El
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.Pp
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Exactly one of either
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.Dv M_WAITOK
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or
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.Dv M_NOWAIT
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must be specified.
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.Pp
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The
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.Fa type
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argument is used to perform statistics on memory usage, and for
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basic sanity checks.
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It can be used to identify multiple allocations.
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The statistics can be examined by
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.Sq vmstat -m .
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.Pp
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A
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.Fa type
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is defined using
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.Vt "struct malloc_type"
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via the
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.Fn MALLOC_DECLARE
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and
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.Fn MALLOC_DEFINE
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macros.
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.Bd -literal -offset indent
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/* sys/something/foo_extern.h */
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MALLOC_DECLARE(M_FOOBUF);
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/* sys/something/foo_main.c */
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MALLOC_DEFINE(M_FOOBUF, "foobuffers", "Buffers to foo data into the ether");
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/* sys/something/foo_subr.c */
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\&...
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buf = malloc(sizeof(*buf), M_FOOBUF, M_NOWAIT);
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.Ed
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.Pp
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In order to use
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.Fn MALLOC_DEFINE ,
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one must include
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.In sys/param.h
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(instead of
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.In sys/types.h )
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and
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.In sys/kernel.h .
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.Sh CONTEXT
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.Fn malloc ,
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.Fn realloc
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and
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.Fn reallocf
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may not be called from fast interrupts handlers.
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When called from threaded interrupts,
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.Fa flags
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must contain
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.Dv M_NOWAIT .
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.Pp
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.Fn malloc ,
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.Fn realloc
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and
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.Fn reallocf
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may sleep when called with
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.Dv M_WAITOK .
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.Fn free
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never sleeps.
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However,
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.Fn malloc ,
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.Fn realloc,
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.Fn reallocf
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and
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.Fn free
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may not be called in a critical section or while holding a spin lock.
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.Pp
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Any calls to
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.Fn malloc
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(even with
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.Dv M_NOWAIT )
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or
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.Fn free
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when holding a
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.Xr vnode 9
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interlock, will cause a LOR (Lock Order Reversal) due to the
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intertwining of VM Objects and Vnodes.
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.Sh IMPLEMENTATION NOTES
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The memory allocator allocates memory in chunks that have size a power
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of two for requests up to the size of a page of memory.
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For larger requests, one or more pages is allocated.
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While it should not be relied upon, this information may be useful for
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optimizing the efficiency of memory use.
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.Sh RETURN VALUES
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The
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.Fn malloc ,
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.Fn realloc ,
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and
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.Fn reallocf
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functions return a kernel virtual address that is suitably aligned for
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storage of any type of object, or
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.Dv NULL
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if the request could not be satisfied (implying that
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.Dv M_NOWAIT
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was set).
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.Sh DIAGNOSTICS
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A kernel compiled with the
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.Dv INVARIANTS
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configuration option attempts to detect memory corruption caused by
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such things as writing outside the allocated area and imbalanced calls to the
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.Fn malloc
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and
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.Fn free
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functions.
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Failing consistency checks will cause a panic or a system console
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message.
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.Sh SEE ALSO
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.Xr vmstat 8 ,
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.Xr contigmalloc 9 ,
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.Xr memguard 9 ,
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.Xr vnode 9
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