Sat Jan 8 18:07:15 2011 UTC ()

CONSIDERATION => CONSIDERATIONS, to be on par with the rest of man pages.


(jym)

diff -r1.11 -r1.12 src/share/man/man9/kmem.9

--- src/share/man/man9/kmem.9 2010/12/02 12:54:13	1.11
+++ src/share/man/man9/kmem.9 2011/01/08 18:07:14	1.12

 @@ -1,275 +1,275 @@
-.\"	$NetBSD: kmem.9,v 1.11 2010/12/02 12:54:13 wiz Exp $
+.\"	$NetBSD: kmem.9,v 1.12 2011/01/08 18:07:14 jym Exp $
 .\"
 .\" Copyright (c)2006 YAMAMOTO Takashi,
 .\" All rights reserved.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
 .\" modification, are permitted provided that the following conditions
 .\" are met:
 .\" 1. Redistributions of source code must retain the above copyright
 .\"    notice, this list of conditions and the following disclaimer.
 .\" 2. Redistributions in binary form must reproduce the above copyright
 .\"    notice, this list of conditions and the following disclaimer in the
 .\"    documentation and/or other materials provided with the distribution.
 .\"
 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
 .\" ------------------------------------------------------------
 .Dd May 16, 2010
 .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 char *
 .Fn kmem_asprintf \
 "const char *fmt" "..."
 .\" ------------------------------------------------------------
 .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_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.
 .\" ------------------------------------------------------------
 .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
 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
 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
 .\" ------------------------------------------------------------
 .Sh CAVEATS
 Neither
 .Fn kmem_alloc
 nor
 .Fn kmem_free
 can be used from interrupt context, from a soft interrupt, or from
 a callout.
 Use
 .Xr pool_cache 9
 in these situations.
 .\" ------------------------------------------------------------
-.Sh SECURITY CONSIDERATION
+.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.