block device level tracing

[Frank Ch. Eigler]

Hi -

If we haven't already, we should write another systemtap sample that
simulates the /proc/sys/vm/block_dump hook in the kernel.  This
consists of two conditional printk's, one in __mark_inode_dirty(), and
one in submit_bio().

Replacing or amending this specialized tracing hack would be yet
another good application of the "marker" technique.  Until then
though, we can hook these functions' internals with ordinary kprobes
and produce analogous data for general probing/tracing.

See http://tinyurl.com/2xu6rn and its referents.

- FChE