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To: "Luoqi Chen" <lchen@briontech.com>
From: Ed L Cashin <ecashin@uga.edu>
Date: Thu, 07 Aug 2003 11:06:46 -0400
In-Reply-To: <AHEKICEOIHLOGINAFIINIEDICBAA.lchen@briontech.com> ("Luoqi
 Chen"'s message of "Wed, 6 Aug 2003 13:45:37 -0700")
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cc: hackers@freebsd.org
Subject: Re: COW and mprotect on non-shared memory
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"Luoqi Chen" <lchen@briontech.com> writes:

[Ed writes]
>> That means that if I do this:
>> 
>>     for (i = 0; i < n; ++i) {
>>       assert(!mprotect(p, pgsiz, PROT_NONE));
>>       assert(!mprotect(p, pgsiz, PROT_READ|PROT_WRITE|PROT_EXEC));
>>       p[i] = i & 0xff;
>>     }
>> 
>> ... I get n minor page faults!  Pretty amazing, but I guess they
>> figured nobody does that.  

...
> The first mprotect() removes the physical mapping from the page
> table, the second mprotect() doesn't do anything because the mapping
> isn't there. So when the page is accessed, a fault is needed to
> insert the mapping back to the page table.

OK, thanks.  I can see that in sys/i386/i386/pmap.c.  It leaves me
wondering what MAP_ENTRY_COW is for, though.

-- 
--Ed L Cashin            |   PGP public key:
  ecashin@uga.edu        |   http://noserose.net/e/pgp/