[$] Keeping COWs in context (a.k.a. anonymous reverse mapping)

The kernel's reverse-mapping machinery is charged with locating thepage-table entries that refer to a given page in memory. The reversemapping of anonymous pages is handled differently than for file-backedpages. The kernel's implementation of reverse mapping for anonymous pagesis, according to Lorenzo Stoakes in his proposalfor a memory-management-track session at the 2026 Linux Storage,Filesystem, Memory Management, and BPF Summit, "a very brokenabstraction", due to its complexity. It also has some performanceproblems. Stoakes was there to present, in raw form, a proposedreplacement that he calls a "COW context".