stop thrashing the page cache

See the README for some extra explanation.

ref:
- http://insights.oetiker.ch/linux/fadvise/ (Improving Linux performance by
  preserving Buffer Cache State)
- http://lwn.net/Articles/449420/ (fadvise: support POSIX_FADV_NOREUSE)
- http://lwn.net/Articles/480930/ (fadvise: support POSIX_FADV_NOREUSE)

Linus' trick sync_file_range trick (cited):
- http://lkml.iu.edu//hypermail/linux/kernel/1005.2/01845.html

We could probably make it easier somehow to do the writeback and discard
thing, but I have had _very_ good experiences with even a rather trivial
file writer that basically used (iirc) 8MB windows, and the logic was very
trivial:

- before writing a new 8M window, do "start writeback"
  (SYNC_FILE_RANGE_WRITE) on the previous window, and do a wait
  (SYNC_FILE_RANGE_WAIT_AFTER) on the window before that.

in fact, in its simplest form, you can do it like this (this is from my
"overwrite disk images" program that I use on old disks):

for (index = 0; index < max_index ;index++) {
  if (write(fd, buffer, BUFSIZE) != BUFSIZE)
    break;
  /* This won't block, but will start writeout asynchronously */
  sync_file_range(fd, index*BUFSIZE, BUFSIZE, SYNC_FILE_RANGE_WRITE);
  /* This does a blocking write-and-wait on any old ranges */
  if (index)
    sync_file_range(fd, (index-1)*BUFSIZE, BUFSIZE, SYNC_FILE_RANGE_WAIT_BEFORE|
      SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER);
  }
  ...

and even if you don't actually do a discard (maybe we should add a
SYNC_FILE_RANGE_DISCARD bit, right now you'd need to do a separate
fadvise(FADV_DONTNEED) to throw it out) the system behavior is pretty nice,
because the heavy writer gets good IO performance _and_ leaves only
easy-to-free pages around after itself.

Linus
utee 1.1.0

- stop thrashing the page cache (configurable)