[Gllug] Pages marked as present but not mapped

Adrian McMenamin adrian at mcmen.demon.co.uk
Mon Aug 29 11:06:26 UTC 2011

I wonder if one of the kernel gurus here can help me with this - I have
rifled through a lot of literature but cannot find a comprehensive

I have written some code (https://github.com/mcmenaminadrian/valext) that
uses ptrace to step through the execution of a Linux program and on every
step query the /proc/PID/pagemap to record how many pages are present.

The code seems to work fine (but if anyone spots an error in it let me
know!) but returns a lot of pages which are marked as present but which
have no page table entries. Typically these are as numerous as pages which
are present and have a PTE.

Now, what I read is that these pages do exist - but nowhere have I read a
decent explanation of what they are. Anyone here able to give me an
explanation or point me towards one?



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