[PATCH] readahead: ->prev_page can overrun the ahead window

[PATCH] readahead: ->prev_page can overrun the ahead window

If get_next_ra_size() does not grow fast enough, ->prev_page can overrun
the ahead window.  This means the caller will read the pages from
->ahead_start + ->ahead_size to ->prev_page synchronously.

Signed-off-by: Oleg Nesterov <[email protected]>
Cc: Steven Pratt <[email protected]>
Cc: Ram Pai <[email protected]>
Cc: Trond Myklebust <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>