Man page 9 : ksize

ksize - get the actual amount of memory allocated for a given object

size_t ksize(const void *  objp );

objp
    Pointer to the object

kmalloc may internally round up allocations and return more memory than requested. ksize can be used to determine the actual amount of memory allocated. The caller may use this additional memory, even though a smaller amount of memory was initially specified with the kmalloc call. The caller must guarantee that objp points to a valid object previously allocated with either kmalloc or kmem_cache_alloc. The object must not be freed during the duration of the call.