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.