fio mmap sequential read 30% regression

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

Comapring with 2.6.30's result, fio mmap sequtial read has
about 30% regression on one of my stoakley machine with 1
JBOD (7 SAS disks) with kernel 2.6.31-rc1.

Every disk has 2 partitions and 4 1-GB files per partition. Start
10 processes per disk to do mmap read sequentinally.

Bisect down to below patch.

ef00e08e26dd5d84271ef706262506b82195e752 is first bad commit
commit ef00e08e26dd5d84271ef706262506b82195e752
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue Jun 16 15:31:25 2009 -0700

    readahead: clean up and simplify the code for filemap page fault readahead
    
    This shouldn't really change behavior all that much, but the single rather
    complex function with read-ahead inside a loop etc is broken up into more
    manageable pieces.
    
    The behaviour is also less subtle, with the read-ahead being done up-front
    rather than inside some subtle loop and thus avoiding the now unnecessary
    extra state variables (ie "did_readaround" is gone).
    
    Fengguang: the code split in fact fixed a bug reported by Pavel Levshin:
    the PGMAJFAULT accounting used to be bypassed when MADV_RANDOM is set, in
    which case the original code will directly jump to no_cached_page reading.


The bisect is stable.

Yanmin