COW and mprotect on non-shared memory

[Ed L Cashin]

"Luoqi Chen" <lchen at 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 at uga.edu        |   http://noserose.net/e/pgp/