[PATCH 00/16]: Fix writeback regressions in 2.6.32

[Stefan Bader]

Impact: 2.6.32 is facing some serious performance regression which are visible
plainly when unmounting filesystems as async writeback degrades to sync in
that case. But there seem to be other less visible problems as well.
I believe this set of patches also would be benefitial to go into upstream
stable.

Fix: This series of patches was picked and backported when needed to apply
to 2.6.32.y (there is another set for 2.6.34.y[1]). It probably does not
solve all currently known problems but testing has shown considerable
improvements. At the beginning of the set there are some patches that were
picked to make the later patches applicable. Only one of them is a bit more
intrusive the others rather trivial.

Testcase: Unmounting a fs that still has larger amounts of data to write back
to disk will take noticably longer time to complete (IIRC might be 30min).
Also general responsiveness was reported to improve. Currently there are no
known regressions reported when testing this backport.

[1] git://kernel.ubuntu.com/smb/linux-2.6.34.y writeback