page_size - Global Variable: Is the virtual page size
extern vm_size_t page_size;
The page_size global variable is the size of a virtual
page on a CPU. A device driver can use this global variable
to partition I/O transfers so that they never cross a
virtual page boundary. You should use this global variable
only when: A subsystem cannot use the DMA mapping routines
(such as, dma_map_alloc, dma_map_load, and so forth) There
is a one-to-one correspondence between the system memory
map and the I/O's view of the system memory space
page_size(9v)
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