Host where KSM appears to save a negative amount of memory

[Chris Webb <chris@arachsys.com>]

We're running KSM on kernel 2.6.39.2 with hosts running a number qemu-kvm
virtual machines, and it has consistently been saving us a useful amount of
RAM.

To monitor the effective amount of memory saved, I've been looking at the
difference between /sys/kernel/mm/ksm/pages_sharing and pages_shared. On a
typical 32GB host, this has been coming out as at least a hundred thousand
or so, which is presumably half to one gigabyte worth of 4k pages.

However, this morning we've spotted something odd - a host where
pages_sharing is smaller than pages_shared, giving a negative saving by the
above calculation:

  # cat /sys/kernel/mm/ksm/pages_sharing
  1099994
  # cat /sys/kernel/mm/ksm/pages_shared
  1761313

I think this means my interpretation of these values must be wrong, as I
presumably can't have more pages being shared than instances of their use!
Can anyone shed any light on what might be going on here for me? Am I
misinterpreting these values, or does this look like it might be an
accounting bug? (If the latter, what useful debug info can I extract from
the system to help identify it?)

Best wishes,

Chris.

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