cvs commit: src/sys/vm memguard.c

[Julian Elischer]

Alfred Perlstein wrote:

>Where can I read more about this feature? :)
>

You have to nail Boscoe to a sheet of 1m x 2m plywood and subject him to
crude borg mind reading techniques.

The upside is that if you succeed we get to do the same to you to find 
out what you learned. :-)

>
>* Bosko Milekic <bmilekic at FreeBSD.org> [050215 14:17] wrote:
>  
>
>>bmilekic    2005-02-15 22:17:07 UTC
>>
>>  FreeBSD src repository
>>
>>  Modified files:
>>    sys/vm               memguard.c 
>>  Log:
>>  Rather than overloading the page->object field like UMA does, use instead
>>  an unused pageq queue reference in the page structure to stash a pointer
>>  to the MemGuard FIFO.  Using the page->object field caused problems
>>  because when vm_map_protect() was called the second time to set
>>  VM_PROT_DEFAULT back onto a set of pages in memguard_map, the protection
>>  in the VM would be changed but the PMAP code would lazily not restore
>>  the PG_RW bit on the underlying pages right away (see pmap_protect()).
>>  So when a page fault finally occured and the VM noticed the faulting
>>  address corresponds to a page that _does_ have write access now, it
>>  would then call into PMAP to set back PG_RW (i386 case being discussed
>>  here).  However, before it got to do that, an assertion on the object
>>  lock not being owned would get triggered, as the object of the faulting
>>  page would need to be locked but was overloaded by MemGuard.  This is
>>  precisely why MemGuard cannot overload page->object.
>>  
>>  Submitted by: Alan Cox (alc@)
>>  
>>  Revision  Changes    Path
>>  1.4       +13 -17    src/sys/vm/memguard.c
>>    
>>
>
>  
>