Shared file mapping (MAP_SHARED)

With MAP_SHARED, updates are visible to other operating-system processes mapping the same region and are carried through to the backing file. With MAP_PRIVATE, updates instead stay private, and later backing file changes have unspecified visibility in that mapping.^[ref]

In the toy model, both shared file mappings initially read backing bytes $x=7,y=8$ at file offsets 8229 and 8230. Both memory pages have been accessed, so their translations are installed. A shared store changes the common page cache contents and creates a dirty memory page eligible for page writeback. That change can be observed before it reaches persistent storage. Cooperating readers need synchronization to know when an update is ready; MAP_SHARED alone supplies no mutual exclusion or indivisible multi-field update.