Locking Program Text and Data

When you specify the text option, the function locks all executable text as loaded for the program, including shared objects (DSOs). (It does not lock segments created with mmap() even when you specify PROT_EXEC to mmap(). Use mpin() to lock executable, mapped segments.)

When you specify the data option, the function locks the default data (heap) and stack segments, and any mapped segments made with MAP_PRIVATE, as they are defined at the time of the call. If you extend these segments after locking them, the newly defined pages are also locked as they are defined.

Although new pages are locked when they are defined, you still should extend these segments to their maximum size while initializing the program. The reason is that it takes time to extend a segment: the kernel must process a page fault and create a new page frame, possibly writing other pages to backing store to make space.

One way to ensure that the full stack is created before it is locked is to call plock() from a function like the function in Example 1-2.

Example 1-2 : Function to Lock Maximum Stack Size

#define MAX_STACK_DEPTH 100000 /* your best guess */
int call_plock()
{
   char dummy[MAX_STACK_DEPTH];
   return plock(PROCLOCK);
}

The plock() function does not lock mapped segments you create with MAP_SHARED. You must lock them individually using mpin(). You need to do this from only one of the processes that shares the segment.