Hands-On System Programming with Linux

Locking memory

We understand that on a virtual memory-based OS, such as Linux, a usermode page can be swapped at any point in time; the Linux kernel memory management code makes these decisions. To the regular application process, this should not matter: any time it attempts to access (read, write, or execute) the page content, the kernel will page it back into RAM, and allow it to use it as though nothing had occurred. This handling is generally called servicing a page fault (there is a lot more to it, but for the purpose of this discussion, this is sufficient), and is completely transparent to the usermode application process.

However, there are some situations where memory pages being paged – written from RAM to swap and vice-versa –