Possible problems with mmap/munmap on FreeBSD ...

[Richard Sharpe]

Hi,

I am having some problems with the tdb package on FreeBSD 4.6.2 and 4.10.

One of the things the above package does is:

   mmap the tdb file to a region of memory
   store stuff in the region (memmov etc).
   when it needs to extend the size of the region {
     munmap the region
     write data at the end of the file
     mmap the region again with a larger size
   }

What I am seeing is that after the munmap the data written to the region
is gone.

However, if I insert an msync before the munmap, everything is nicely
coherent. This seems odd (in the sense that it works without the msync
under Linux).

The region is mmapped with:

   mmap(NULL, tdb->map_size,
        PROT_READ|(tdb->read_only? 0:PROT_WRITE),
        MAP_SHARED|MAP_FILE, tdb->fd, 0);

What I notice is that all the calls to mmap return the same address.

A careful reading of the man pages for mmap and munmap does not suggest
that I am doing anything wrong.

Is it possible that FreeBSD is deferring flushing the dirty data, and then
forgets to do it when the same starting address is used etc?

Regards
-----
Richard Sharpe, rsharpe[at]richardsharpe.com, rsharpe[at]samba.org,
sharpe[at]ethereal.com, http://www.richardsharpe.com