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KMEM(9)                    Kernel Developer's Manual                   KMEM(9)

NAME
     kmem - kernel wired memory allocator

SYNOPSIS
     #include <sys/kmem.h>

     void *
     kmem_alloc(size_t size, km_flag_t kmflags);

     void *
     kmem_zalloc(size_t size, km_flag_t kmflags);

     void
     kmem_free(void *p, size_t size);

     void *
     kmem_intr_alloc(size_t size, km_flag_t kmflags);

     void *
     kmem_intr_zalloc(size_t size, km_flag_t kmflags);

     void
     kmem_intr_free(void *p, size_t size);

     char *
     kmem_asprintf(const char *fmt, ...);

     options KMEM_SIZE
     options KMEM_REDZONE
     options KMEM_GUARD

DESCRIPTION
     kmem_alloc() allocates kernel wired memory.  It takes the following
     arguments.

     size     Specify the size of allocation in bytes.

     kmflags  Either of the following:

              KM_SLEEP    If the allocation cannot be satisfied immediately,
                          sleep until enough memory is available.  If KM_SLEEP
                          is specified, then the allocation cannot fail.

              KM_NOSLEEP  Don't sleep.  Immediately return NULL if there is
                          not enough memory available.  It should only be used
                          when failure to allocate will not have harmful,
                          user-visible effects.

                          Use of KM_NOSLEEP is strongly discouraged as it can
                          create transient, hard to debug failures that occur
                          when the system is under memory pressure.

                          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.

     The contents of allocated memory are uninitialized.

     Unlike Solaris, kmem_alloc(0, flags) is illegal.

     kmem_zalloc() is the equivalent of kmem_alloc(), except that it
     initializes the memory to zero.

     kmem_asprintf() functions as the well known asprintf() function, but
     allocates memory using 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 kmem_free().

     kmem_free() frees kernel wired memory allocated by kmem_alloc() or
     kmem_zalloc() so that it can be used for other purposes.  It takes the
     following arguments.

     p        The pointer to the memory being freed.  It must be the one
              returned by kmem_alloc() or kmem_zalloc().

     size     The size of the memory being freed, in bytes.  It must be the
              same as the size argument used for kmem_alloc() or kmem_zalloc()
              when the memory was allocated.

     Freeing NULL is illegal.

     kmem_intr_alloc(), kmem_intr_zalloc() and 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,
     pool_cache(9) should be used for memory allocation from interrupt
     context.

NOTES
     Making 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
     kmem_free() may also block.

     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:

       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));
             }

OPTIONS
   KMEM_SIZE
     Kernels compiled with the KMEM_SIZE option ensure the size given in
     kmem_free() matches the actual allocated size.  On 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 kmem_free().  Any mismatch
     triggers a panic.

     KMEM_SIZE is enabled by default on DIAGNOSTIC and DEBUG.

   KMEM_REDZONE
     Kernels compiled with the KMEM_REDZONE option add a dynamic pattern of
     two bytes at the end of each allocated buffer, and check this pattern
     when freeing to ensure the caller hasn't written outside the requested
     area.  This option does not introduce a significant performance impact,
     but has two drawbacks: it only catches write overflows, and catches them
     only on kmem_free().

     KMEM_REDZONE is enabled by default on DIAGNOSTIC.

   KMEM_GUARD
     Kernels compiled with the KMEM_GUARD option perform CPU intensive sanity
     checks on kmem operations.  It adds additional, very high overhead
     runtime verification to kmem operations.  It must be enabled with
     KMEM_SIZE.

     KMEM_GUARD tries to catch the following types of bugs:

        Overflow at time of occurrence, by means of a guard page.  An
         unmapped guard page sits immediately after the requested area; a
         read/write overflow therefore triggers a page fault.

        Underflow at kmem_free(), by using KMEM_SIZE's registered size.  If
         an underflow occurs, the size stored by KMEM_SIZE will be
         overwritten, which means that when freeing, the kernel will spot the
         mismatch.

        Use-after-free at time of occurrence.  When freeing, the memory is
         unmapped, and depending on the value of kmem_guard_depth, the kernel
         will more or less delay the recycling of that memory.  Which means
         that any ulterior read/write access to the memory will trigger a page
         fault, given it hasn't been recycled yet.

     To enable it, boot the system with the -d option, which causes the
     debugger to be entered early during the kernel boot process.  Issue
     commands such as the following:

     db> w kmem_guard_depth 0t30000
     db> c

     This instructs kmem_guard to queue up to 60000 (30000*2) pages of
     unmapped KVA to catch use-after-free type errors.  When 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:

        It has a severe impact on performance.

        It is best used on a 64-bit machine with lots of RAM.

     KMEM_GUARD is enabled by default on DEBUG.

RETURN VALUES
     On success, kmem_alloc() and kmem_zalloc() return a pointer to allocated
     memory.  Otherwise, NULL is returned.

CODE REFERENCES
     The kmem subsystem is implemented within the file sys/kern/subr_kmem.c.

SEE ALSO
     intro(9), memoryallocators(9), percpu(9), pool_cache(9), uvm_km(9)

CAVEATS
     Neither kmem_alloc() nor kmem_free() can be used from interrupt context,
     from a soft interrupt, or from a callout.  Use pool_cache(9) in these
     situations.

SECURITY CONSIDERATIONS
     As the memory allocated by 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.

NetBSD 8.0                     February 28, 2016                    NetBSD 8.0