man/man9/kmalloc.9

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.\" $NetBSD: malloc.9,v 1.3 1996/11/11 00:05:11 lukem Exp $
.\" $FreeBSD: src/share/man/man9/malloc.9,v 1.42 2005/02/22 17:20:20 brueffer Exp $
.\"
.Dd November 21, 2017
.Dt KMALLOC 9
.Os
.Sh NAME
.Nm kmalloc ,
.Nm kmalloc_cachealign ,
.Nm kfree ,
.Nm krealloc ,
.Nm kmalloc_raise_limit ,
.Nm MALLOC_DEFINE ,
.Nm MALLOC_DECLARE
.Nd kernel memory management routines
.Sh SYNOPSIS
.In sys/types.h
.In sys/malloc.h
.Ft void *
.Fn kmalloc "unsigned long size" "struct malloc_type *type" "int flags"
.Ft void *
.Fn kmalloc_cachealign "unsigned long size" "struct malloc_type *type" "int flags"
.Ft void
.Fn kfree "void *addr" "struct malloc_type *type"
.Ft void *
.Fn krealloc "void *addr" "unsigned long size" "struct malloc_type *type" "int flags"
.Ft void
.Fn kmalloc_raise_limit "struct malloc_type *type" "size_t bytes"
.Fn MALLOC_DECLARE type
.In sys/param.h
.In sys/malloc.h
.In sys/kernel.h
.Fn MALLOC_DEFINE type shortdesc longdesc
.Sh DESCRIPTION
The
.Fn kmalloc
function allocates uninitialized memory in kernel address space for an
object whose size is specified by
.Fa size .
.Fn kmalloc_cachealign
function is same as
.Fn kmalloc
except that the allocated memory will be cache line size aligned.
.Pp
The
.Fn kfree
function releases memory at address
.Fa addr
that was previously allocated by
.Fn kmalloc
for re-use.
The memory is not zeroed.
The kernel implementation of
.Fn kfree
does not allow
.Fa addr
to be
.Dv NULL .
.Pp
The
.Fn krealloc
function changes the size of the previously allocated memory referenced by
.Fa addr
to
.Fa size
bytes.
The contents of the memory are unchanged up to the lesser of the new and
old sizes.
Note that the returned value may differ from
.Fa addr .
If the requested memory cannot be allocated,
.Dv NULL
is returned and the memory referenced by
.Fa addr
is valid and unchanged.
If
.Fa addr
is
.Dv NULL ,
the
.Fn krealloc
function behaves identically to
.Fn kmalloc
for the specified size.
.Pp
.Fn kmalloc_raise_limit
is used to increase the internal pool limit to
.Fa bytes .
Under most of the cases
the default internal pool limit should be more than enough,
so this function is currently rarely used and must be used with care.
.Pp
Unlike its standard C library counterpart
.Pq Xr malloc 3 ,
the kernel version takes two more arguments.
The
.Fa flags
argument further qualifies
.Fn kmalloc Ns 's
operational characteristics as follows:
.Bl -tag -width indent
.It Dv M_ZERO
Causes the allocated memory to be set to all zeros.
.It Dv M_NOWAIT
Causes
.Fn kmalloc
and
.Fn krealloc ,
to return
.Dv NULL
if the request cannot be immediately fulfilled due to resource shortage.
Note that
.Dv M_NOWAIT
is required when running in an interrupt context.
.It Dv M_WAITOK
Indicates that it is OK to wait for resources.
If the request cannot be immediately fulfilled, the current process is put
to sleep to wait for resources to be released by other processes.
Before the internal pool limit is reached,
the
.Fn kmalloc
and
.Fn krealloc ,
functions cannot return
.Dv NULL
if
.Dv M_WAITOK
is specified.
If the internal pool limit is reached and
.Dv M_NULLOK
is not specified along with
.Dv M_WAITOK ,
the system will panic.
If the internal pool limit is reached and
.Dv M_NULLOK
is specified along with
.Dv M_WAITOK ,
the
.Fn kmalloc
and
.Fn krealloc ,
functions return
.Dv NULL
instead of panicing the system.
.It Dv M_INTWAIT
Indicates
.Fn kmalloc
to dig into the system's reserved free pages looking for enough room to
perform the allocation.
This is typically used in interrupts where you cannot afford
.Fn kmalloc
to fail.
Before the internal pool limit is reached,
the
.Fn kmalloc
and
.Fn krealloc ,
functions cannot return
.Dv NULL
if
.Dv M_INTWAIT
is specified.
If the internal pool limit is reached and
.Dv M_NULLOK
is not specified along with
.Dv M_INTWAIT ,
the system will panic.
If the internal pool limit is reached and
.Dv M_NULLOK
is specified along with
.Dv M_INTWAIT ,
the
.Fn kmalloc
and
.Fn krealloc ,
functions return
.Dv NULL
instead of panicing the system.
.It Dv M_USE_RESERVE
Indicates that the system can dig into its reserve in order to obtain the
requested memory.
.It Dv M_POWEROF2
Rounds up the size to the nearest power of 2.
.It Dv M_NULLOK
This flag is usually specified along with
.Dv M_WAITOK
or
.Dv M_INTWAIT ,
so when the interal pool limit is reached,
.Fn kmalloc
and
.Fn krealloc ,
functions will not panic the system,
instead,
.Dv NULL
will be returned.
This flag is usually used on the kernel code path that is triggered by
user space programs' requests.
.El
.Pp
Exactly one of either
.Dv M_WAITOK ,
.Dv M_INTWAIT
or
.Dv M_NOWAIT
must be specified.
.Pp
The
.Fa type
argument is used to perform statistics on memory usage, and for
basic sanity checks.
It can be used to identify multiple allocations.
The statistics can be examined by
.Sq vmstat -m .
.Pp
A
.Fa type
is defined using the
.Va malloc_type_t
typedef via the
.Fn MALLOC_DECLARE
and
.Fn MALLOC_DEFINE
macros.
.Bd -literal -offset indent
/* sys/something/foo_extern.h */

MALLOC_DECLARE(M_FOOBUF);

/* sys/something/foo_main.c */

MALLOC_DEFINE(M_FOOBUF, "foobuffers", "Buffers to foo data into the ether");

/* sys/something/foo_subr.c */

\&...
buf = kmalloc(sizeof *buf, M_FOOBUF, M_NOWAIT);

.Ed
.Sh IMPLEMENTATION NOTES
The memory allocator allocates memory in chunks that have size a power
of two for requests up to the size of a page of memory.
For larger requests, one or more pages is allocated.
The allocated memory will be at least 8 bytes aligned.
While it should not be relied upon, this information may be useful for
optimizing the efficiency of memory use.
.Sh RETURN VALUES
The
.Fn kmalloc
and
.Fn krealloc ,
functions return a kernel virtual address that is suitably aligned for
storage of any type of object, or
.Dv NULL
if the request could not be satisfied (implying that
.Dv M_NOWAIT
or
.Dv M_NULLOK
was set).
.Sh DIAGNOSTICS
A kernel compiled with the
.Dv INVARIANTS
configuration option attempts to detect memory corruption caused by
such things as writing outside the allocated area and imbalanced calls to the
.Fn kmalloc
and
.Fn kfree
functions.
Failing consistency checks will cause a panic or a system console
message.
.Sh SEE ALSO
.Xr vmstat 8 ,
.Xr contigmalloc 9 ,
.Xr kstrdup 9 ,
.Xr memory 9 ,
.Xr vnode 9