Windows® Internals, Sixth Edition, Part 2

Reference count The number of references to this page. The reference count is incremented when a page is first added to a working set and/or when the page is locked in memory for I/O (for example, by a device driver). The reference count is decremented when the share count becomes 0 or when pages are unlocked from memory. When the share count becomes 0, the page is no longer owned by a working set. Then, if the reference count is also zero, the PFN database entry that describes the page is updated to add the page to the free, standby, or modified list.

Type The type of page represented by this PFN. (Types include active/valid, standby, modified, modified-no-write, free, zeroed, bad, and transition.)

Flags The information contained in the flags field is shown in Table 10-17.

Priority The priority associated with this PFN, which will determine on which standby list it will be placed.

Original PTE contents All PFN database entries contain the original contents of the PTE that pointed to the page (which could be a prototype PTE). Saving the contents of the PTE allows it to be restored when the physical page is no longer resident. PFN entries for AWE allocations are exceptions; they store the AWE reference count in this field instead.

PFN of PTE Physical page number of the page table page containing the PTE that points to this page.

Color Besides being linked together on a list, PFN database entries use an additional field to link physical pages by “color,” which is the page’s NUMA node number.

Flags A second flags field is used to encode additional information on the PTE. These flags are described in Table 10-18.

Table 10-17. Flags Within PFN Database Entries

Flag

Meaning

Write in progress

Indicates that a page write operation is in progress. The first DWORD contains the address of the event object that will be signaled when the I/O is complete.

Modified state

Indicates whether the page was modified. (If the page was modified, its contents must be saved to disk before removing it from memory.)

Read in progress

Indicates that an in-page operation is in progress for the page. The first DWORD contains the address of the event object that will be signaled when the I/O is complete.

Rom

Indicates that this page comes from the computer’s firmware or another piece of read-only memory such as a device register.

In-page error

Indicates that an I/O error occurred during the in-page operation on this page. (In this case, the first field in the PFN contains the error code.)

Kernel stack

Indicates that this page is being used to contain a kernel stack. In this case, the PFN entry contains the owner of the stack and the next stack PFN for this thread.

Removal requested

Indicates that the page is the target of a remove (due to ECC/scrubbing or hot memory removal).

Parity error

Indicates that the physical page contains parity or error correction control errors.

Table 10-18. Secondary Flags Within PFN Database Entries

Flag

Meaning

PFN image verified

The code signature for this PFN (contained in the cryptographic signature catalog for the image being backed by this PFN) has been verified.

AWE allocation

This PFN backs an AWE allocation.

Prototype PTE

Indicates that the PTE referenced by the PFN entry is a prototype PTE. (For example, this page is shareable.)

The remaining fields are specific to the type of PFN. For example, the first PFN in Figure 10-42 represents a page that is active and part of a working set. The share count field represents the number of PTEs that refer to this page. (Pages marked read-only, copy-on-write, or shared read/write can be shared by multiple processes.) For page table pages, this field is the number of valid and transition PTEs in the page table. As long as the share count is greater than 0, the page isn’t eligible for removal from memory.

The working set index field is an index into the process working set list (or the system or session working set list, or zero if not in any working set) where the virtual address that maps this physical page resides. If the page is a private page, the working set index field refers directly to the entry in the working set list because the page is mapped only at a single virtual address. In the case of a shared page, the working set index is a hint that is guaranteed to be correct only for the first process that made the page valid. (Other processes will try to use the same index where possible.) The process that initially sets this field is guaranteed to refer to the proper index and doesn’t need to add a working set list hash entry referenced by the virtual address into its working set hash tree. This guarantee reduces the size of the working set hash tree and makes searches faster for these particular direct entries.