Windows® Internals, Sixth Edition, Part 2

When the system initializes, the MiInitializeDynamicVa function sets up the basic dynamic ranges (the ranges currently supported are described in Table 10-9) and sets the available virtual address to all available kernel space. It then initializes the address space ranges for boot loader images, process space (hyperspace), and the HAL through the MiIntializeSystemVaRange function, which is used to set hard-coded address ranges. Later, when nonpaged pool is initialized, this function is used again to reserve the virtual address ranges for it. Finally, whenever a driver loads, the address range is relabeled to a driver image range (instead of a boot loaded range).

After this point, the rest of the system virtual address space can be dynamically requested and released through MiObtainSystemVa (and its analogous MiObtainSessionVa) and MiReturnSystemVa. Operations such as expanding the system cache, the system PTEs, nonpaged pool, paged pool, and/or special pool; mapping memory with large pages; creating the PFN database; and creating a new session all result in dynamic virtual address allocations for a specific range. Each time the kernel virtual address space allocator obtains virtual memory ranges for use by a certain type of virtual address, it updates the MiSystemVaType array, which contains the virtual address type for the newly allocated range. The values that can appear in MiSystemVaType are shown in Table 10-9.

Table 10-9. System Virtual Address Types

Region

Description

Limitable

MiVaSessionSpace (0x1)

Addresses for session space

Yes

MiVaProcessSpace (0x2)

Addresses for process address space

No

MiVaBootLoaded (0x3)

Addresses for images loaded by the boot loader

No

MiVaPfnDatabase (0x4)

Addresses for the PFN database

No

MiVaNonPagedPool (0x5)

Addresses for the nonpaged pool

Yes

MiVaPagedPool (0x6)

Addresses for the paged pool

Yes

MiVaSpecialPool (0x7)

Addresses for the special pool

No

MiVaSystemCache (0x8)

Addresses for the system cache

Yes

MiVaSystemPtes (0x9)

Addresses for system PTEs

Yes

MiVaHal (0xA)

Addresses for the HAL

No

MiVaSessionGlobalSpace (0xB)

Addresses for session global space

No

MiVaDriverImages (0xC)

Addresses for loaded driver images

No

Although the ability to dynamically reserve virtual address space on demand allows better management of virtual memory, it would be useless without the ability to free this memory. As such, when paged pool or the system cache can be shrunk, or when special pool and large page mappings are freed, the associated virtual address is freed. (Another case is when the boot registry is released.) This allows dynamic management of memory depending on each component’s use. Additionally, components can reclaim memory through MiReclaimSystemVa, which requests virtual addresses associated with the system cache to be flushed out (through the dereference segment thread) if available virtual address space has dropped below 128 MB. (Reclaiming can also be satisfied if initial nonpaged pool has been freed.)

In addition to better proportioning and better management of virtual addresses dedicated to different kernel memory consumers, the dynamic virtual address allocator also has advantages when it comes to memory footprint reduction. Instead of having to manually preallocate static page table entries and page tables, paging-related structures are allocated on demand. On both 32-bit and 64-bit systems, this reduces boot-time memory usage because unused addresses won’t have their page tables allocated. It also means that on 64-bit systems, the large address space regions that are reserved don’t need to have their page tables mapped in memory, which allows them to have arbitrarily large limits, especially on systems that have little physical RAM to back the resulting paging structures.

EXPERIMENT: Querying System Virtual Address Usage