[Gllug] Pages marked as present but not mapped

Adrian McMenamin adrian at mcmen.demon.co.uk
Wed Aug 31 16:29:32 UTC 2011

On Wed, August 31, 2011 12:48 am, Nix wrote:
> On 30 Aug 2011, Christian Smith said:
>> What I suspect is happening is that you're reading entries for pages
>> that
>> are not present (else the present bit would be 1) but are not swapped.
> Agreed. Some of this may be text pages which haven't been paged in: some
> may be due to malloc() overcommitment.
These pages *are* marked as present, at least in the /proc file system.

What they do not have is any page table entry (or swap reference).

Would I be correct in saying/thinking, they are/could be, areas of the
process virtual memory that have been reserved for, say, more pages of the
backing file to be read in, but that the file itself has not been read in
- hence although the space is reserved, there is nothing in it and when at
attempt is made to reference those addresses, the appropriate page the
address_space will be read in from backing store and a page table entry
fixed up.

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