[PATCH] laptop mode

From: Bart Samwel <bart@samwel.tk>

Adds /proc/sys/vm/laptop-mode: a special knob which says "this is a laptop".
In this mode the kernel will attempt to avoid spinning disks up.

Algorithm: the idea is to hold dirty data in memory for a long time, but to
flush everything which has been accumulated if the disk happens to spin up
for other reasons.

- Whenever a disk request completes (read or write), schedule a timer a few
  seconds hence.  If the timer was already pending, reset it to a few seconds
  hence.

- When the timer expires, write back the whole world.  We use
  sync_filesystems() for this because it will force ext3 journal commits as
  well.

- In balance_dirty_pages(), kick off background writeback when we hit the
  high threshold (dirty_ratio), not when we hit the low threshold.  This has
  the effect of causing "lumpy" writeback which is something I spent a year
  fixing, but in laptop mode, it is desirable.

- In try_to_free_pages(), only kick pdflush if the VM is getting into
  distress: we want to keep scanning for clean pages, deferring writeback.

- In page reclaim, avoid writing back the odd random dirty page off the
  LRU: only start I/O if the scanning is working harder.

The effect is to perform a sync() a few seconds after all I/O has ceased.

The value which was written into /proc/sys/vm/laptop-mode determines, in
seconds, the delay between the final I/O and the flush.

Additionally, the patch adds tools which help answer the question "why the
heck does my disk spin up all the time?".  The user may set
/proc/sys/vm/block_dump to a non-zero value and the kernel will print out
information which will identify the process which is performing disk reads or
which is dirtying pagecache.

The user should probably disable syslogd before setting block-dump.

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