vm-region supports creating nested regions of virtual memory, each with their
own memory protection restrictions.
vm-objects can be mapped into a vm-region, making the underlying memory
accessible. all mappings are lazy: page tables are not updated until the
mapped memory is accessed.
vm-object can be used to demand-allocate non-contiguous physical memory, and
will provide an api for userspace programs to do the same. unless a vm-object
is created in-place (i.e. to represent a specific area of physical memory),
its memory pages are only allocated when the object is mapped AND someone
attempts to access the memory.
when the sector coverage mode is set to free, the loop that initialises the vm_page structs for free and reserved pages is limited to the same upper bound that is used to calculate the sector size and count.
by default, the sector map created under the sparse model now only extends to the last non-reserved page frame, any reserved page frames afterwards are ignored.
previously, sparse would attempt to create a smaller number of larger sectors on systems with lots of reserved memory, often causing an out-of-memory condition. the reserved memory ratio calculation now compares reserved memory to free memory, rather than to the address of the last byte in physical memory. this improved heuristic means sparse is now better at choosing an appropriate sector size, allowing sparse to operate on systems with high amounts of reserved memory.
allocating a large power-of-2 block with memblock may cause the base pointer of a region to exceed the limit pointer after it has been aligned during the free region scan in do_alloc().
* the page array now only extends up to the last non-reserved memory frame.
* rather than memset'ing the whole array to zero, we only initialise p_flags
for each page in vm_page_array_init, and then leave it to group_pages_into_block
to initialise the other parts of vm_page_t.
if kmalloc is called with count=N before vm_bootstrap is finished,
the request will be fulfilled using memblock_alloc.
if N is a power of 2, the returned pointer will be aligned to
an N-byte boundary.
caches with object size N, where N is a power of two, will naturally
align their objects on N-byte boundaries.
all other caches will align to an 8-byte boundary by default.
