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.\"     $NetBSD: kmem.9,v 1.2 2009/08/03 20:10:16 rmind Exp $
.\"
.\" Copyright (c)2006 YAMAMOTO Takashi,
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.\" ------------------------------------------------------------
.Dd August 3, 2009
.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"
.\" ------------------------------------------------------------
.Pp
.Cd "options DEBUG"
.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 KM_SLEEP
If the allocation cannot be satisfied immediately, sleep until enough
memory is available.
.It 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_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.
.\" ------------------------------------------------------------
.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.
.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
Kernels compiled with the
.Dv DEBUG
option perform CPU intensive sanity checks on kmem operations,
and include the
.Dv kmguard
facility which can be enabled at runtime.
.Pp
.Dv kmguard
adds additional, very high overhead runtime verification to kmem operations.
To enable it, boot the system with the
.Fl d
option, which causes the debugger to be entered early during the kernel
boot process.
Issue commands such as the following:
.Bd -literal
db\*[Gt] w kmem_guard_depth 0t30000
db\*[Gt] c
.Ed
.Pp
This instructs
.Dv kmguard
to queue up to 60000 (30000*2) pages of unmapped KVA to catch
use-after-free type errors.
When
.Fn kmem_free
is called, memory backing a freed item is unmapped and the kernel VA
space pushed onto a FIFO.
The VA space will not be reused until another 30k items have been freed.
Until reused the kernel will catch invalid accesses and panic with a page fault.
Limitations:
.Bl -bullet
.It
It has a severe impact on performance.
.It
It is best used on a 64-bit machine with lots of RAM.
.It
Allocations larger than PAGE_SIZE bypass the
.Dv kmguard
facility.
.El
.Pp
kmguard tries to catch the following types of bugs:
.Bl -bullet
.It
Overflow at time of occurrence, by means of a guard page.
.It
Underflow at
.Fn kmem_free ,
by using a canary value.
.It
Invalid pointer or size passed, at
.Fn kmem_free .
.El
.Sh RETURN VALUES
On success,
.Fn kmem_alloc
and
.Fn kmem_zalloc
return a pointer to allocated memory.
Otherwise,
.Dv NULL
is returned.
.\" ------------------------------------------------------------
.Sh CODE REFERENCES
This section describes places within the
.Nx
source tree where actual code implementing the
.Nm
subsystem
can be found.
All pathnames are relative to
.Pa /usr/src .
.Pp
The
.Nm
subsystem is implemented within the file
.Pa sys/kern/subr_kmem.c .
.\" ------------------------------------------------------------
.Sh SEE ALSO
.Xr intro 9 ,
.Xr memoryallocators 9 ,
.Xr pool_cache 9
.\" ------------------------------------------------------------
.Sh CAVEATS
.Fn kmem_alloc
cannot be used from interrupt context, from a soft interrupt, or from
a callout.
Use
.Xr pool_cache 9
in these situations.
.\" ------------------------------------------------------------
.Sh SECURITY CONSIDERATION
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.