st subtract this from the process's `write' accounting.  But
  that means that some processes would be reported to have done negative
  amounts of write IO, which is silly.

  So we just report the raw number and punt this decision up to userspace.

Now, we _could_ account for writes at the physical I/O level.  But

- This would require that we track memory-dirtying tasks at the per-page
  level (would require a new pointer in struct page).

- It would mean that IO statistics for a process are usually only available
  long after that process has exitted.  Which means that we probably cannot
  communicate this info via taskstats.

This patch:

Wire up the kernel-private data structures and the accessor functions to
manipulate them.

Cc: Jay Lan <jlan@sgi.com>
Cc: Shailabh Nagar <nagar@watson.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Chris Sturtivant <csturtiv@sgi.com>
Cc: Tony Ernst <tee@sgi.com>
Cc: Guillaume Thouvenin <guillaume.thouvenin@bull.net>
Cc: David Wright <daw@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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