[Bug 270810] munmap does not always sync the underlying file

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270810

--- Comment #6 from geoffrey@dommett.com ---
The smallest test program that I have replicated this with is below.
When run on a linux machine the problem does not occur, when writing to the nfs
server running bsd.
When run on a bsd machine, the problem seems to occur if the number of
instances run is sufficient that the machine has insufficient ram that it must
page between the files and the in memory copy to complete the operation. When
running 50 instances is enough on the machine in front of me with 8gb of ram
about 30% of them are corrupt at the end. It does not occur running just a few
instances. 
Running 50 instances of this test on a bsd machine with 96Gb ram, 1 of them is
corrupt. This same machine exhibits a lot of corruption when 300+ Gb of files
are mapped.
I think all times I have encountered this have been when the total files mapped
exceed the physical ram of the machine.

leaving in or commenting out the msync, fsync and ftruncate at the end does not
stop the file corruption. 

#include <stdio.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>

int main (int argv , char** argc )
{
int fl=open(argc[1] , O_RDWR | O_CREAT , 0644 ) ;
lseek(fl,1000000000 , SEEK_SET);
write(fl, "", 1);
int * ttv = (int*)  mmap(0,100000000 , PROT_WRITE | PROT_READ ,MAP_SHARED, fl ,
0) ;


for (int j=0 ; j<10 ; j++) {
for (int ii = j ; ii<100000000/4 ; ii+=10 )
        ttv[ii]=ii ;
}
//msync(ttv  , 10000000000, MS_SYNC ) ;

//fsync(fl) ;
//ftruncate(fl ,9000000000) ;
close(fl) ;
}

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