512K readahead size with thrashing safe readahead v2

Andrew,

This enlarges the default readahead size from 128K to 512K. To avoid possible regressions, also do
- scale down readahead size on small device and small memory
- thrashing safe context readahead
- add readahead tracing/stats support to help expose possible problems

Besides, the patchset also includes several algorithm updates:
- no start-of-file readahead after lseek
- faster radix_tree_next_hole()/radix_tree_prev_hole()
- pagecache context based mmap read-around

Changes since v1:
- update mmap read-around heuristics (Thanks to Nick Piggin)
- radix_tree_lookup_leaf_node() for the pagecache based mmap read-around
- use __print_symbolic() to show readahead pattern names (Thanks to Steven Rostedt)
- scale down readahead size proportional to system memory (Thanks to Matt Mackall)
- add readahead size kernel parameter (by Nikanth Karthikesan)
- add comments from Christian Ehrhardt

Changes since RFC:
- move the lenthy intro text to individual patch changelogs
- treat get_capacity()==0 as uninitilized value (Thanks to Vivek Goyal)
- increase readahead size limit for small devices (Thanks to Jens Axboe)
- add fio test results by Vivek Goyal

[PATCH 01/15] readahead: limit readahead size for small devices [PATCH 02/15] readahead: retain inactive lru pages to be accessed soon [PATCH 03/15] readahead: bump up the default readahead size [PATCH 04/15] readahead: make default readahead size a kernel parameter [PATCH 05/15] readahead: limit readahead size for small memory systems [PATCH 06/15] readahead: replace ra->mmap_miss with ra->ra_flags [PATCH 07/15] readahead: thrashing safe context readahead [PATCH 08/15] readahead: record readahead patterns
[PATCH 09/15] readahead: add tracing event
[PATCH 10/15] readahead: add /debug/readahead/stats
[PATCH 11/15] readahead: dont do start-of-file readahead after lseek() [PATCH 12/15] radixtree: introduce radix_tree_lookup_leaf_node() [PATCH 13/15] radixtree: speed up the search for hole
[PATCH 14/15] readahead: reduce MMAP_LOTSAMISS for mmap read-around [PATCH 15/15] readahead: pagecache context based mmap read-around

 Documentation/kernel-parameters.txt |    4 
 block/blk-core.c                    |    3 
 block/genhd.c                       |   24 +
 fs/fuse/inode.c                     |    2 
 fs/read_write.c                     |    3 
 include/linux/fs.h                  |   64 +++
 include/linux/mm.h                  |    8 
 include/linux/radix-tree.h          |    2 
 include/trace/events/readahead.h    |   78 ++++
 lib/radix-tree.c                    |   94 ++++-
 mm/Kconfig                          |   13 
 mm/filemap.c                        |   30 +
 mm/readahead.c                      |  459 ++++++++++++++++++++++----
 13 files changed, 680 insertions(+), 104 deletions(-)

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