Subject: Observations on our VM system problem
To: None <tech-kern@NetBSD.ORG>
From: None <ghudson@MIT.EDU>
List: tech-kern
Date: 03/01/1996 06:12:27
I wrote a test program to illustrate the canonical NetBSD VM system
problem (vnode-backed VM pages can get out of sync with the filesystem
and stay that way indefinitely).  I've inclueded the test program at
the end of this message.  It should print "Hello.\nThere.\n" if it
works properly; it actually prints "Hello.\n\0\0\0\0\0\0\0".  Notice
that an msync() after the second mmap() does not avert the problem (so
the patch I recently sent to the guy on netbsd-help is useless).

I'm trying to figure out why msync() doesn't help.  It's easy to
determine that it's supposed to work; it attempts to free all the
physical pages associated with the vm objects corresponding to the
memory region you specify.  Some curious things I noticed while

	* sys_msync() calls vm_map_clean() which calls
	  vm_object_page_remove() to remove all physical pages from
	  the vm object.  When I watched this happen under DDB, there
	  were apparently no physical pages in the vm object's memq
	  (there were zero iterations of the loop in

	* I tried breaking on vm_fault() after the invocation of
	  sys_msync(), and my breakpoint was not triggered until AFTER
	  "Hello.\n\0\0\0\0\0\0\0" was printed.  This means no page
	  fault occurred when addr was referenced after the msync()
	  call.  Thus, there must have been a physical page set up to
	  take the address reference, even though it wasn't found in
	  the vm object's memq.

Can anyone shed some light on this?  It could mean that vm_map_clean()
is using incorrect logic and winds up cleaning the wrong objects, or
it could mean that the vm structures are getting corrupt and there's a
page corresponding to this vm object which simply isn't being cleaned.

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

int main()
    char buf[128] = "/tmp/test-vm.XXXX", c;
    int fd, fd_hold;
    caddr_t addr, addr_hold;

    /* Create a file. */
    fd = mkstemp(buf);
    write(fd, "Hello.\n", 7);

    /* mmap() the file and bring the page into the VM cache. */
    fd_hold = open(buf, O_RDONLY, 0);
    addr_hold = mmap(NULL, 7, PROT_READ, 0, fd_hold, 0);
    c = *addr_hold;

    /* Modify the file through the buffer cache. */
    fd = open(buf, O_RDWR | O_APPEND, 0);
    write(fd, "There.\n", 7);

    /* Now mmap() the file again, and display its contents. */
    fd = open(buf, O_RDONLY, 0);
    addr = mmap(NULL, 14, PROT_READ, 0, fd, 0);
    c = *addr;
    msync(addr, 14);
    write(1, addr, 14);
    munmap(addr, 14);