If the page is locked

The only thing we can do about locked pages is to roll the page over, and to try to free some other page, we'll try to free this page again at the next pass.

Locked pages can be under VM critical sections, they could be getting freed by another CPU under us for example. The VM serializes against itself using the locked bitflag.

Or more generally the locked pages can be under I/O, like with swapouts/swapins/pageins.