[$] An ioctl() call to detect memory writes

It is the kernel's business to know when a process's memory has beenwritten to; among other things, this knowledge is needed to determine whichpages can be immediately reclaimed or to properly write dirty pages to backing store.Sometimes, though, user space also needs access to this information in areliable and fast manner. Thispatch series from Muhammad Usama Anjum adds a new ioctl() callfor this purpose; using it requires repurposing an existing system call inan unusual way, though.