PTE modified bit emulation trap

Alan Cox alc at
Fri May 15 16:54:33 UTC 2009

John Baldwin wrote:
> On Thursday 14 May 2009 12:42:39 pm Alan Cox wrote:
>> Dag-Erling Smørgrav wrote:
>>> [from -alpha, -hackers]
>>> Dag-Erling Smørgrav <des at> writes:
>>>> Coverity complains about the lack of error checking in the following
>>>> code in sys/kern/kern_sysctl.c, around line 1390:
>>>> 	/*
>>>> 	 * Touch all the wired pages to avoid PTE modified
>>>> 	 * bit emulation traps on Alpha while holding locks
>>>> 	 * in the sysctl handler.
>>>> 	 */
>>>> 	for (i = (wiredlen + PAGE_SIZE - 1) / PAGE_SIZE,
>>>> 	    cp = req->oldptr; i > 0; i--, cp += PAGE_SIZE) {
>>>> 		copyin(cp, &dummy, 1);
>>>> 		copyout(&dummy, cp, 1);
>>>> 	}
>>>> Since Alpha is dead, can we remove this, or is it still needed for other
>>>> platforms?
>>> kmacy suggested you might be the right person to ask...  the conclusion
>>> so far is that it *might* be necessary on sparc64 and / or mips.
>> I think that this code may no longer be needed, but I want to 
>> double-check.  I faced a related problem implementing superpages 
>> support, so I introduced an additional "access type" parameter to 
>> pmap_enter().  This parameter was specifically intended to allow a 
>> pmap_enter() implementation to preset the PTE's modified bit.  I think 
>> that the simulated page fault that occurs on vslock()-style wiring 
>> passes "write access" to pmap_enter().  If so, then it's just a matter 
>> of tweaking the MIPS or any other pmap_enter() to actually do something 
>> with the "access type" parameter.  Currently, only the architectures 
>> that implement the pmap-level support for superpages, i.e., amd64 and 
>> i386, do anything with this parameter.
> Then it sounds like the code should definitely be removed and that if any 
> problems do crop up, they can be fixed in pmap_enter() instead.

I've had a chance to verify what I said above, so you can remove the 
code.  I don't think that sparc64 will require any changes, but MIPS 
needs a two-line change to pmap_enter().  I'll see that the change gets 


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