The kernel’s page cache holds pages from files in RAM, allowing those
pages 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 is
present in the page cache at any given time; the proposed
cachestat() system call from Nhat Pham is the latest in a long
series of attempts to make that information available.

Source: LWN.net – [$] Checking page-cache status with cachestat()