[$] Checking page-cache status with cachestat()

The kernel's page cache holds pages from files in RAM, allowing thosepages to be accessed without expensive trips to persistent storage.Applications are normally entirely unaware of the page cache's operation;it speeds things up and that is all that matters. Some applications,though, can benefit from knowledge about how much of a given file ispresent in the page cache at any given time; the proposedcachestat() system call from Nhat Pham is the latest in a longseries of attempts to make that information available.