Updated: 2021/Apr/14


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, ...);

     char *
     kmem_strdupsize(const char *str, size_t *size, km_flag_t kmflags);

     char *
     kmem_strdup(const char *str, km_flag_t kmflags);

     char *
     kmem_strndup(const char *str, size_t manxlen, km_flag_t kmflags);

     void
     kmem_strfree(char *str);

     void *
     kmem_tmpbuf_alloc(size_t size, void *stackbuf, size_t stackbufsize,
         km_flag_t kmflags);

     void
     kmem_tmpbuf_free(void *p, size_t size, void *stackbuf);

     options KMEM_SIZE

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.

     The kmem_strdupsize() function is a utility function that can be used to
     copy the string in the str argument to a new buffer allocated using
     kmem_alloc() and optionally return the size of the allocation (the length
     of the string plus the trailing NUL) in the size argument if that is not
     NULL.

     The kmem_strdup() function is a simplified version of kmem_strdupsize()
     that does not return the size of the allocation.

     The kmem_strndup() function is variation of kmem_strdup() that copies at
     most maxlen characters from the string str always NUL terminating the
     copied string.

     The kmem_strfree() function can be used to free a NUL terminated string
     computing the length of the string using strlen(3) and adding one for the
     NUL and then using kmem_free().

     The 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 kmem_alloc().  The kmem_tmpbuf_free() function compares
     the result of a previous call to kmem_tmpbuf_alloc() and frees the memory
     using kmem_free() if it is not the specified stack buffer.

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.

RETURN VALUES
     On success, kmem_alloc(), kmem_asprintf(), kmem_intr_alloc(),
     kmem_intr_zalloc(), kmem_strdupsize(), 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
     The kmem_alloc(), kmem_asprintf(), kmem_free(), kmem_strdupsize(),
     kmem_strfree(), and kmem_zalloc() functions cannot 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 9.99                    January 24, 2021                    NetBSD 9.99