.\"	$NetBSD: kmem.9,v 1.28 2021/03/06 14:44:02 rin Exp $
.\"
.\" Copyright (c)2006 YAMAMOTO Takashi,
.\" All rights reserved.
.\"
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.\" ------------------------------------------------------------
.Dd January 24, 2021
.Dt KMEM 9
.Os
.\" ------------------------------------------------------------
.Sh NAME
.Nm kmem
.Nd kernel wired memory allocator
.\" ------------------------------------------------------------
.Sh SYNOPSIS
.In sys/kmem.h
.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
.Ft void *
.Fn kmem_alloc \
"size_t size" "km_flag_t kmflags"
.Ft void *
.Fn kmem_zalloc \
"size_t size" "km_flag_t kmflags"
.Ft void
.Fn kmem_free \
"void *p" "size_t size"
.\" ---
.Ft void *
.Fn kmem_intr_alloc \
"size_t size" "km_flag_t kmflags"
.Ft void *
.Fn kmem_intr_zalloc \
"size_t size" "km_flag_t kmflags"
.Ft void
.Fn kmem_intr_free \
"void *p" "size_t size"
.\" ---
.Ft char *
.Fn kmem_asprintf \
"const char *fmt" "..."
.\" ---
.Ft char *
.Fn kmem_strdupsize \
"const char *str" "size_t *size" "km_flag_t kmflags"
.Ft char *
.Fn kmem_strdup \
"const char *str" "km_flag_t kmflags"
.Ft char *
.Fn kmem_strndup \
"const char *str" "size_t manxlen" "km_flag_t kmflags"
.Ft void
.Fn kmem_strfree \
"char *str"
.\" ------------------------------------------------------------
.Ft void *
.Fn kmem_tmpbuf_alloc \
"size_t size" "void *stackbuf" "size_t stackbufsize" "km_flag_t kmflags"
.Ft void
.Fn kmem_tmpbuf_free \
"void *p" "size_t size" "void *stackbuf"
.\" ------------------------------------------------------------
.Pp
.Cd "options KMEM_SIZE"
.Sh DESCRIPTION
.Fn kmem_alloc
allocates kernel wired memory.
It takes the following arguments.
.Bl -tag -width kmflags
.It Fa size
Specify the size of allocation in bytes.
.It Fa kmflags
Either of the following:
.Bl -tag -width KM_NOSLEEP
.It Dv KM_SLEEP
If the allocation cannot be satisfied immediately, sleep until enough
memory is available.
If
.Dv KM_SLEEP
is specified, then the allocation cannot fail.
.It Dv KM_NOSLEEP
Don't sleep.
Immediately return
.Dv NULL
if there is not enough memory available.
It should only be used when failure to allocate will not have harmful,
user-visible effects.
.Pp
.Bf -symbolic
Use of
.Dv KM_NOSLEEP
is strongly discouraged as it can create transient, hard to debug failures
that occur when the system is under memory pressure.
.Ef
.Pp
In situations where it is not possible to sleep, for example because locks
are held by the caller, the code path should be restructured to allow the
allocation to be made in another place.
.El
.El
.Pp
The contents of allocated memory are uninitialized.
.Pp
Unlike Solaris, kmem_alloc(0, flags) is illegal.
.Pp
.\" ------------------------------------------------------------
.Fn kmem_zalloc
is the equivalent of
.Fn kmem_alloc ,
except that it initializes the memory to zero.
.Pp
.\" ------------------------------------------------------------
.Fn kmem_asprintf
functions as the well known
.Fn asprintf
function, but allocates memory using
.Fn kmem_alloc .
This routine can sleep during allocation.
The size of the allocated area is the length of the returned character string, plus one (for the NUL terminator).
This must be taken into consideration when freeing the returned area with
.Fn kmem_free .
.Pp
.\" ------------------------------------------------------------
.Fn kmem_free
frees kernel wired memory allocated by
.Fn kmem_alloc
or
.Fn kmem_zalloc
so that it can be used for other purposes.
It takes the following arguments.
.Bl -tag -width kmflags
.It Fa p
The pointer to the memory being freed.
It must be the one returned by
.Fn kmem_alloc
or
.Fn kmem_zalloc .
.It Fa size
The size of the memory being freed, in bytes.
It must be the same as the
.Fa size
argument used for
.Fn kmem_alloc
or
.Fn kmem_zalloc
when the memory was allocated.
.El
.Pp
Freeing
.Dv NULL
is illegal.
.Pp
.\" ------------------------------------------------------------
.Fn kmem_intr_alloc ,
.Fn kmem_intr_zalloc
and
.Fn kmem_intr_free
are the equivalents of the above kmem routines which can be called
from the interrupt context.
These routines are for the special cases.
Normally,
.Xr pool_cache 9
should be used for memory allocation from interrupt context.
.Pp
The
.Fn kmem_strdupsize
function is a utility function that can be used to copy the string in the
.Fa str
argument to a new buffer allocated using
.Fn kmem_alloc
and optionally return the size of the allocation (the length of the string
plus the trailing
.Dv NUL )
in the
.Fa size
argument if that is not
.Dv NULL .
.Pp
The
.Fn kmem_strdup
function is a simplified version of
.Fn kmem_strdupsize
that does not return the size of the allocation.
.Pp
The
.Fn kmem_strndup
function is variation of
.Fn kmem_strdup
that copies at most
.Fa maxlen
characters from the string
.Fa str
always NUL terminating the copied string.
.Pp
The
.Fn kmem_strfree
function can be used to free a
.Dv NUL
terminated string computing the length of the string using
.Xr strlen 3
and adding one for the
.Dv NUL
and then using
.Fn kmem_free .
.Pp
The
.Fn kmem_tmpbuf_alloc
function is a utility function for allocating memory for temporary
use, where allocation on the stack is desirable, but only up to a
certain size.
If the requested size fits within the specified stack buffer, the
stack buffer is returned.
Otherwise, memory is allocated with
.Fn kmem_alloc .
The
.Fn kmem_tmpbuf_free
function compares the result of a previous call to
.Fn kmem_tmpbuf_alloc
and frees the memory using
.Fn kmem_free
if it is not the specified stack buffer.
.\" ------------------------------------------------------------
.Sh NOTES
Making
.Dv KM_SLEEP
allocations while holding mutexes or reader/writer locks is discouraged, as the
caller can sleep for an unbounded amount of time in order to satisfy the
allocation.
This can in turn block other threads that wish to acquire locks held by the
caller.
It should be noted that
.Fn kmem_free
may also block.
.Pp
For some locks this is permissible or even unavoidable.
For others, particularly locks that may be taken from soft interrupt context,
it is a serious problem.
As a general rule it is better not to allow this type of situation to develop.
One way to circumvent the problem is to make allocations speculative and part
of a retryable sequence.
For example:
.Bd -literal
  retry:
        /* speculative unlocked check */
        if (need to allocate) {
                new_item = kmem_alloc(sizeof(*new_item), KM_SLEEP);
        } else {
                new_item = NULL;
        }
        mutex_enter(lock);
        /* check while holding lock for true status */
        if (need to allocate) {
                if (new_item == NULL) {
                        mutex_exit(lock);
                        goto retry;
                }
                consume(new_item);
                new_item = NULL;
        }
        mutex_exit(lock);
        if (new_item != NULL) {
                /* did not use it after all */
                kmem_free(new_item, sizeof(*new_item));
        }
.Ed
.\" ------------------------------------------------------------
.Sh OPTIONS
.Ss KMEM_SIZE
Kernels compiled with the
.Dv KMEM_SIZE
option ensure the size given in
.Fn kmem_free
matches the actual allocated size.
On
.Fn kmem_alloc ,
the kernel will allocate an additional contiguous kmem page of eight
bytes in the buffer, will register the allocated size in the first kmem
page of that buffer, and will return a pointer to the second kmem page
in that same buffer.
When freeing, the kernel reads the first page, and compares the
size registered with the one given in
.Fn kmem_free .
Any mismatch triggers a panic.
.Pp
.Dv KMEM_SIZE
is enabled by default on
.Dv DIAGNOSTIC .
.Sh RETURN VALUES
On success,
.Fn kmem_alloc ,
.Fn kmem_asprintf ,
.Fn kmem_intr_alloc ,
.Fn kmem_intr_zalloc ,
.Fn kmem_strdupsize ,
and
.Fn kmem_zalloc
return a pointer to allocated memory.
Otherwise,
.Dv NULL
is returned.
.\" ------------------------------------------------------------
.Sh CODE REFERENCES
The
.Nm
subsystem is implemented within the file
.Pa sys/kern/subr_kmem.c .
.\" ------------------------------------------------------------
.Sh SEE ALSO
.Xr intro 9 ,
.Xr memoryallocators 9 ,
.Xr percpu 9 ,
.Xr pool_cache 9 ,
.Xr uvm_km 9
.\" ------------------------------------------------------------
.Sh CAVEATS
The
.Fn kmem_alloc ,
.Fn kmem_asprintf ,
.Fn kmem_free ,
.Fn kmem_strdupsize ,
.Fn kmem_strfree ,
and
.Fn kmem_zalloc
functions cannot be used from interrupt context, from a soft interrupt,
or from a callout.
Use
.Xr pool_cache 9
in these situations.
.\" ------------------------------------------------------------
.Sh SECURITY CONSIDERATIONS
As the memory allocated by
.Fn kmem_alloc
is uninitialized, it can contain security-sensitive data left by its
previous user.
It is the caller's responsibility not to expose it to the world.