Windows® Internals, Sixth Edition, Part 2

Finally, the !vm command in the kernel debugger shows the basic memory management information available through the memory-related performance counters. This command can be useful if you’re looking at a crash dump or hung system. Here’s an example of its output from a 4-GB Windows client system:1: kd> !vm *** Virtual Memory Usage *** Physical Memory: 851757 ( 3407028 Kb) Page File: \??\C:\pagefile.sys Current: 3407028 Kb Free Space: 3407024 Kb Minimum: 3407028 Kb Maximum: 4193280 Kb Available Pages: 699186 ( 2796744 Kb) ResAvail Pages: 757454 ( 3029816 Kb) Locked IO Pages: 0 ( 0 Kb) Free System PTEs: 370673 ( 1482692 Kb) Modified Pages: 9799 ( 39196 Kb) Modified PF Pages: 9798 ( 39192 Kb) NonPagedPool Usage: 0 ( 0 Kb) NonPagedPoolNx Usage: 8735 ( 34940 Kb) NonPagedPool Max: 522368 ( 2089472 Kb) PagedPool 0 Usage: 17573 ( 70292 Kb) PagedPool 1 Usage: 2417 ( 9668 Kb) PagedPool 2 Usage: 0 ( 0 Kb) PagedPool 3 Usage: 0 ( 0 Kb) PagedPool 4 Usage: 28 ( 112 Kb) PagedPool Usage: 20018 ( 80072 Kb) PagedPool Maximum: 523264 ( 2093056 Kb) Session Commit: 6218 ( 24872 Kb) Shared Commit: 18591 ( 74364 Kb) Special Pool: 0 ( 0 Kb) Shared Process: 2151 ( 8604 Kb) PagedPool Commit: 20031 ( 80124 Kb) Driver Commit: 4531 ( 18124 Kb) Committed pages: 179178 ( 716712 Kb) Commit limit: 1702548 ( 6810192 Kb) Total Private: 66073 ( 264292 Kb) 0a30 CCC.exe 11078 ( 44312 Kb) 0548 dwm.exe 6548 ( 26192 Kb) 091c MOM.exe 6103 ( 24412 Kb) ...

We will describe many of the details of the output of this command later in this chapter.

Services Provided by the Memory Manager

The memory manager provides a set of system services to allocate and free virtual memory, share memory between processes, map files into memory, flush virtual pages to disk, retrieve information about a range of virtual pages, change the protection of virtual pages, and lock the virtual pages into memory.

Like other Windows executive services, the memory management services allow their caller to supply a process handle indicating the particular process whose virtual memory is to be manipulated. The caller can thus manipulate either its own memory or (with the proper permissions) the memory of another process. For example, if a process creates a child process, by default it has the right to manipulate the child process’s virtual memory. Thereafter, the parent process can allocate, deallocate, read, and write memory on behalf of the child process by calling virtual memory services and passing a handle to the child process as an argument. This feature is used by subsystems to manage the memory of their client processes. It is also essential for implementing debuggers because debuggers must be able to read and write to the memory of the process being debugged.

Most of these services are exposed through the Windows API. The Windows API has three groups of functions for managing memory in applications: heap functions (Heapxxx and the older interfaces Localxxx and Globalxxx, which internally make use of the Heapxxx APIs), which may be used for allocations smaller than a page; virtual memory functions, which operate with page granularity (Virtualxxx); and memory mapped file functions (CreateFileMapping, CreateFileMappingNuma, MapViewOfFile, MapViewOfFileEx, and MapViewOfFileExNuma). (We’ll describe the heap manager later in this chapter.)

The memory manager also provides a number of services (such as allocating and deallocating physical memory and locking pages in physical memory for direct memory access [DMA] transfers) to other kernel-mode components inside the executive as well as to device drivers. These functions begin with the prefix Mm. In addition, though not strictly part of the memory manager, some executive support routines that begin with Ex are used to allocate and deallocate from the system heaps (paged and nonpaged pool) as well as to manipulate look-aside lists. We’ll touch on these topics later in this chapter in the section Kernel-Mode Heaps (System Memory Pools)).

Large and Small Pages