Among the many other things crammed into the page structure
that is used
to represent a page of memory in the kernel is a set of flags to track the
state of the page. These flags have been in short supply for some time;
LWN looked at the problem nearly ten years
ago. Jérôme Glisse ran a session during the memory-management track of the
2019 Linux Storage, Filesystem, and Memory-Management Summit to explore
ways of making some flags available for new uses. While there may be some
easily available bits in the field that holds the page flags, obtaining a
significant number of them may be tricky.

Source: LWN.net – [$] The search for available page flags