The system pager requires access to the swap file at all times, even when an application has locked the volume containing the file. To ensure access, the system always gives the system pager the opportunity to accept or reject a lock request.
If the system grants a lock to a process, the requesting process must ensure that data for the swap file remains unchanged and that the pager can safely read from or write to the swap file at any time. In particular, the process must not change the allocation chain of the swap file, and if it moves the swap file's directory entry, it must ensure that the path to it is always consistent. Failure to observe these guidelines can result in a system crash.
An application can determine the directory entry and allocation chain of the swap file by retrieving the path of the file using Find Swap File (Interrupt 21h Function 440Dh Minor Code 6Eh).
The system permits the swap file to grow or shrink in the level 1 and 2 locks. Because of this, an application should call Get Lock Flag State (Interrupt 21h Function 440Dh Minor Code 6Ch) after obtaining a level 3 lock to determine if anything on the disk has changed as the result of the swap file growing or shrinking.