[cout]

The reason memory access can block is because it can cause the page fault handler to be invoked (https://www.kernel.org/doc/gorman/html/understand/understand...). There are many reasons the page fault handler might cause the process to block. The kernel made need to swap the page in from disk, or it might be a copy-on-write page that was just written to.

To copy the page, the kernel needs to first find a free page frame. If there are no free page frames, the kernel will attempt to reclaim pages that are in use (https://www.kernel.org/doc/gorman/html/understand/understand...). This may cause in process-mapped pages being swapped to disk, but it also may result in disk writeback activity (https://lwn.net/Articles/396561/). In either case, control cannot be returned to the process until there is a free page to map into the process.