Understanding the Linux Kernel, Second Edition

Transferring Swap Pages

Transferring swap pages wouldn’t be so complicated if there weren’t so many race conditions and other potential hazards to guard against. Here are some of the things that have to be checked regularly:

• The process that owns a page may terminate while the page is being swapped in or out.

• Another process may be in the middle of swapping in a page that the current one is trying to swap out (or vice versa).

Like any other disk access type, I/O data transfers for swap pages are blocking operations. Therefore, the kernel must take care to avoid simultaneous transfers involving the same page frame, the same page slot, or both.

Race conditions can be avoided on the page frame through the mechanisms discussed in Chapter 13. Specifically, before starting an I/O operation on the page frame, the kernel waits until its PG_locked flag is off. When the function returns, the page frame lock has been acquired, and therefore no other kernel control path can access the page frame’s contents during the I/O operation.

But the state of the page slot must also be tracked. The PG_locked flag of the page descriptor is used once again to ensure exclusive access to the page slot involved in the I/O data transfer. Before starting an I/O operation on a swap page, the kernel checks that the page frame involved is included in the swap cache; if not, it adds the page frame into the swap cache. Let’s suppose some process tries to swap in a page while the same page is currently being transferred. ...