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From: shekita@provolone.cs.wisc.edu (E Shekita)
Newsgroups: comp.os.mach
Subject: simple question on memory objects and copy-on-write
Message-ID: <10859@spool.cs.wisc.edu>
Date: 18 Jul 90 22:50:33 GMT
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I have a question about memory objects and copy-on-write (COW).

Let's suppose I have a memory object server that is in charge of 
paging object X. Further suppose that when a memory_object_data_request() 
comes in for a page in X, my memory object server does the following:

    (1) vm_allocate()'s a page P in its virtual address space
    (2) reads the requested page from disk into P
    (3) provides P to Mach via memory_object_data_provided()
    (4) vm_deallocate()'s P

The question is: will P be mapped COW in step (3), or will Mach copy 
P immediately in step (3)? Obviously it would be nice if P was mapped 
COW in step (3), because then an unnecessary copy is avoided.

Initially, I though that P will definitely be mapped COW, just as in, 
say, vm_copy(). However, P is backed by a different memory object 
(the default pager) than X, and I have recently been told that a physical 
page cannot be shared by two different memory objects. Consequently, I'm 
not so sure what happens.... Can anyone at CMU set me straight?

thanks in advance -- Gene