How to find out which pages were copied-on-write?

[Lutz Vieweg <lkv@isg.de>]

Hi,

in an application that MAP_PRIVATEly mmap()s a file it would
be quite helpful for me to find out which pages have been
copied-on-write.

I found that mincore() does a similar thing by reporting which
pages are currently residing in physical memory, but what
I want to know is which pages differ from the original file
image on disk.

Can you recommend a way to do that? (does not need to be
portable beyond Linux)

Alternatively, it would be sufficient if I could turn
a private mapping into a shared one (and possibly do an
msync() afterwards if I need to make sure the changes
have been written out). Would such a feature need a
lot of effort to implement?


Yet another feature that I could use if it were available:
A "copy-on-read"-mapping. There, a page would become a private
copy of a process once _another_ process wrote data to the
corresponding file location. But I suspect that feature
could be very hard to implement...

Regards,

Lutz Vieweg