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From: shekita@provolone.cs.wisc.edu (E Shekita)
Newsgroups: comp.os.mach
Subject: bug in Mach 2.5 in copy-on-write?
Message-ID: <10871@spool.cs.wisc.edu>
Date: 20 Jul 90 19:05:20 GMT
Sender: news@spool.cs.wisc.edu
Lines: 40

There seems to be a bug in Mach 2.5 with copy-on-write (COW),
where it forgets to unmark a page as COW once the virtual copy
of the page has taken a COW fault. Here's an example that 
picks up the bug:

-----------------------------------------------------------------------
int     i;			/* index */
char*   source_page;            /* vm_read() source */
char*   destination_page;	/* vm_read() destination */

vm_allocate(task_self(), (vm_address_t*) &source_page, vm_page_size, TRUE);

for (i = 0; i < 100; i++) {
    /*
     * Update the source page. This shouldn't trigger a COW fault.
     */
    source_page[0] = 'a'; 

    /*
     * Virtually copy the source page to the destination page. 
     * After the copy, the source and destination pages should
     * both be marked as COW.
     */
    vm_read(task_self(), (vm_address_t) source_page, vm_page_size, 
	   (vm_address_t*) &destination_page, &readCount);

    /*
     * Update the destination page. This should trigger a COW fault,
     * but it should also unmark the source page as COW.
     */
    destination_page[0] = 'b';
}
---------------------------------------------------------------------------

If you run this loop and calculate the number of COW faults
that occured in it using vm_statistics(), you'll find that the
number of COW faults is 199. Shouldn't it be on the order 
of 100?... Any comments from the CMU folks?

Gene