Bug in kernel 2.6.31, Slow wb_kupdate writeout

[Chad Talbott <ctalbott@google.com>]

I run a simple workload on a 4GB machine which dirties a few largish
inodes like so:

# seq 10 | xargs -P0 -n1 -i\{} dd if=/dev/zero of=/tmp/dump\{}
bs=1024k count=100

While the dds are running data is written out at disk speed.  However,
once the dds have run to completion and exited there is ~500MB of
dirty memory left.  Background writeout then takes about 3 more
minutes to clean memory at only ~3.3MB/s.  When I explicitly sync, I
can see that the disk is capable of 40MB/s, which finishes off the
files in ~10s. [1]

An interesting recent-ish change is "writeback: speed up writeback of
big dirty files."  When I revert the change to __sync_single_inode the
problem appears to go away and background writeout proceeds at disk
speed.  Interestingly, that code is in the git commit [2], but not in
the post to LKML. [3]  This is may not be the fix, but it makes this
test behave better.

Thanks,
Chad

[1] I've plotted the dirty memory from /proc/meminfo and disk write
speed from iostat at
http://sites.google.com/site/cwtlinux/2-6-31-writeback-bug
[2] git commit:
http://mirror.celinuxforum.org/gitstat/commit-detail.php?commit=8bc3be2751b4f74ab90a446da1912fd8204d53f7
[3] LKML post: http://marc.info/?l=linux-kernel&m=119131601130372&w=2

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