mm: document why we don't set PageReadahead

If the page is already in cache, we don't set PageReadahead on it.

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Cc: Chao Yu <[email protected]>
Cc: Cong Wang <[email protected]>
Cc: Darrick J. Wong <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Gao Xiang <[email protected]>
Cc: Jaegeuk Kim <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Joseph Qi <[email protected]>
Cc: Junxiao Bi <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Miklos Szeredi <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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