-/*
- * Readahead design.
- *
- * The fields in struct file_ra_state represent the most-recently-executed
- * readahead attempt:
- *
- * start: Page index at which we started the readahead
- * size: Number of pages in that read
- * Together, these form the "current window".
- * Together, start and size represent the `readahead window'.
- * prev_index: The page which the readahead algorithm most-recently inspected.
- * It is mainly used to detect sequential file reading.
- * If page_cache_readahead sees that it is again being called for
- * a page which it just looked at, it can return immediately without
- * making any state changes.
- * offset: Offset in the prev_index where the last read ended - used for
- * detection of sequential file reading.
- * ahead_start,
- * ahead_size: Together, these form the "ahead window".
- * ra_pages: The externally controlled max readahead for this fd.
- *
- * When readahead is in the off state (size == 0), readahead is disabled.
- * In this state, prev_index is used to detect the resumption of sequential I/O.
- *
- * The readahead code manages two windows - the "current" and the "ahead"
- * windows. The intent is that while the application is walking the pages
- * in the current window, I/O is underway on the ahead window. When the
- * current window is fully traversed, it is replaced by the ahead window
- * and the ahead window is invalidated. When this copying happens, the
- * new current window's pages are probably still locked. So
- * we submit a new batch of I/O immediately, creating a new ahead window.
- *
- * So:
- *
- * ----|----------------|----------------|-----
- * ^start ^start+size
- * ^ahead_start ^ahead_start+ahead_size
- *
- * ^ When this page is read, we submit I/O for the
- * ahead window.
- *
- * A `readahead hit' occurs when a read request is made against a page which is
- * the next sequential page. Ahead window calculations are done only when it
- * is time to submit a new IO. The code ramps up the size agressively at first,
- * but slow down as it approaches max_readhead.
- *
- * Any seek/ramdom IO will result in readahead being turned off. It will resume
- * at the first sequential access.
- *
- * There is a special-case: if the first page which the application tries to
- * read happens to be the first page of the file, it is assumed that a linear
- * read is about to happen and the window is immediately set to the initial size
- * based on I/O request size and the max_readahead.
- *
- * This function is to be called for every read request, rather than when
- * it is time to perform readahead. It is called only once for the entire I/O
- * regardless of size unless readahead is unable to start enough I/O to satisfy
- * the request (I/O request > max_readahead).
- */
-