Windows® Internals, Sixth Edition, Part 2

Unknown The PTE is zero, or the page table doesn’t yet exist (the page directory entry that would provide the physical address of the page table contains zero). In both cases, the memory manager pager must examine the virtual address descriptors (VADs) to determine whether this virtual address has been committed. If so, page tables are built to represent the newly committed address space. (See the discussion of VADs later in the chapter.) If not (if the page is reserved or hasn’t been defined at all), the page fault is reported as an access violation exception.

Prototype PTEs

If a page can be shared between two processes, the memory manager uses a software structure called prototype page table entries (prototype PTEs) to map these potentially shared pages. For page-file-backed sections, an array of prototype PTEs is created when a section object is first created; for mapped files, portions of the array are created on demand as each view is mapped. These prototype PTEs are part of the segment structure, described at the end of this chapter.

When a process first references a page mapped to a view of a section object (recall that the VADs are created only when the view is mapped), the memory manager uses the information in the prototype PTE to fill in the real PTE used for address translation in the process page table. When a shared page is made valid, both the process PTE and the prototype PTE point to the physical page containing the data. To track the number of process PTEs that reference a valid shared page, a counter in its PFN database entry is incremented. Thus, the memory manager can determine when a shared page is no longer referenced by any page table and thus can be made invalid and moved to a transition list or written out to disk.

When a shareable page is invalidated, the PTE in the process page table is filled in with a special PTE that points to the prototype PTE entry that describes the page, as shown in Figure 10-28.

Figure 10-28. Structure of an invalid PTE that points to the prototype PTE

Thus, when the page is later accessed, the memory manager can locate the prototype PTE using the information encoded in this PTE, which in turn describes the page being referenced. A shared page can be in one of six different states as described by the prototype PTE entry:

Active/valid The page is in physical memory as a result of another process that accessed it.

Transition The desired page is in memory on the standby or modified list (or not on any list).

Modified-no-write The desired page is in memory and on the modified-no-write list. (See Table 10-19.)

Demand zero The desired page should be satisfied with a page of zeros.

Page file The desired page resides within a page file.

Mapped file The desired page resides within a mapped file.

Although the format of these prototype PTE entries is the same as that of the real PTE entries described earlier, these prototype PTEs aren’t used for address translation—they are a layer between the page table and the page frame number database and never appear directly in page tables.

By having all the accessors of a potentially shared page point to a prototype PTE to resolve faults, the memory manager can manage shared pages without needing to update the page tables of each process sharing the page. For example, a shared code or data page might be paged out to disk at some point. When the memory manager retrieves the page from disk, it needs only to update the prototype PTE to point to the page’s new physical location—the PTEs in each of the processes sharing the page remain the same (with the valid bit clear and still pointing to the prototype PTE). Later, as processes reference the page, the real PTE will get updated.

Figure 10-29 illustrates two virtual pages in a mapped view. One is valid, and the other is invalid. As shown, the first page is valid and is pointed to by the process PTE and the prototype PTE. The second page is in the paging file—the prototype PTE contains its exact location. The process PTE (and any other processes with that page mapped) points to this prototype PTE.

Figure 10-29. Prototype page table entries

In-Paging I/O

In-paging I/O occurs when a read operation must be issued to a file (paging or mapped) to satisfy a page fault. Also, because page tables are pageable, the processing of a page fault can incur additional I/O if necessary when the system is loading the page table page that contains the PTE or the prototype PTE that describes the original page being referenced.