Fast patch for Severe I/O performance regression 2.6.6 to 2.6.7 or2.6.8-rc3

[Mr. Berkley Shands]

in 2.6.8-rc3/mm/readahead.c line 475 (about label do_io:)
#if 0
           ra->next_size = min(average , (unsigned long)max);
#endif

the comment for the above is here after an lseek. The lseek IS inside 
the window, but the code will always
destroy the window and start again. The above patch corrects the 
performance problem,
but it would be better to do nothing if the lseek is still within the 
read ahead window.

berkley
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