LU-13669 llite: make readahead aware of hints

LU-13669 llite: make readahead aware of hints

Calling madvise(MADV_SEQUENTIAL) and madvise(MADV_RANDOM) sets the
VM_SEQ_READ and VM_RAND_READ hints in vma->vm_flags.  These should
be used to guide the Lustre readahead for better performance.

Disable the kernel readahead for mmap() pages and use the llite
readahead instead.  There was also a bug in ll_fault0() that would
set both VM_SEQ_READ and VM_RAND_READ at the same time, which was
confusing the detection of the VM_SEQ_READ case, since VM_RAND_READ
was being checked first.

This changes the readahead for mmap from submitting mostly 4KB RPCs
to a large number of 1MB RPCs for the application profiled:

  llite.*.read_ahead_stats     before        patched
  ------------------------     ------        -------
  hits                           2408         135924 samples [pages]
  misses                        34160           2384 samples [pages]

  osc.*.rpc_stats           read before    read patched
  ---------------          -------------  --------------
  pages per rpc            rpcs   % cum%   rpcs   % cum%
     1:                    6542  95  95     351  55  55
     2:                     224   3  99      76  12  67
     4:                      32   0  99      28   4  72
     8:                       2   0  99       9   1  73
    16:                      25   0  99      32   5  78
    32:                       0   0  99       8   1  80
    64:                       0   0  99       5   0  80
   128:                       0   0  99      15   2  83
   256:                       2   0  99     102  16  99
   512:                       0   0  99       0   0  99
  1024:                       1   0 100       3   0 100

Readahead hit rate improved from 6% to 98%, and 4KB RPCs dropped from
95% to 55% and 1MB+ RPCs increased from 0% to 16% (79% of all pages).

Add debug to ll_file_mmap(), ll_fault() and ll_fault_io_init() to
allow tracing VMA state functions for future IO optimizations.

Fixes: 62ef9c949753 ("add 2.6.27 kernel support")
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I4bbb028db05b21ae01dafe6a7bea398e9b74d8a4
Reviewed-on: https://review.whamcloud.com/41228
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>