Subject: Re: Observations on our VM system problem
To: Frank van der Linden <email@example.com>
From: Greg Hudson <ghudson@MIT.EDU>
Date: 03/01/1996 14:36:30
> Seems to me that you need to specify MAP_SHARED in the mmap() calls..
This shouldn't be necessary. The VM system will assume MAP_PRIVATE if
you don't specify a flag, and the comments say:
* Copy-on-write of file. Two flavors.
* MAP_COPY is true COW, you essentially get a snapshot of
* the region at the time of mapping. MAP_PRIVATE means only
* that your changes are not reflected back to the object.
* Changes made by others will be seen.
Since I'm never actually touching the mapped regions, doing an msync()
of a private mapping ought to bring the page in sync with the
However, I did try changing my test program to use MAP_SHARED; now it
works if and only if you msync() after the second mmap() call. So
there's apparently a bug relating to the handling of MAP_PRIVATE
mappings and msync().
Note that lib/libc/db/recno/rec_open() uses a MAP_PRIVATE mapping,
explicitly. I can still reproduce the vi problem with msync() calls
after that mmap() (and after the mmap() in usr.bin/vi/ex/ex_tag.c).