[$] Sharing page tables with mshare()

The Linux kernel allows processes to share pages in memory, but the pagetables used to control that sharing are not, themselves, shared; as aresult, processes sharing memory maintain duplicate copies of thepage-table data. Normally this duplication imposes little overhead, butthere are situations where it can hurt. At the 2022 Linux Storage,Filesystem, Memory-management and BPF Summit (LSFMM), Khaled Aziz(remotely) and Matthew Wilcox led a session to discuss a proposed mechanismto allow those page tables to be shared between cooperating processes.