Date: Fri, 15 May 2009 11:24:07 -0500 From: Alan Cox <alc@cs.rice.edu> To: John Baldwin <jhb@freebsd.org>, =?UTF-8?B?RGFnLUVybGluZyBTbcO4cmdyYXY=?= <des@des.no> Cc: alc@freebsd.org, arch@freebsd.org, freebsd-arch@freebsd.org Subject: Re: PTE modified bit emulation trap Message-ID: <4A0D9727.6030703@cs.rice.edu> In-Reply-To: <200905150804.36977.jhb@freebsd.org> References: <86tz3o4lb9.fsf@ds4.des.no> <86prec4kwj.fsf@ds4.des.no> <4A0C49FF.1070707@cs.rice.edu> <200905150804.36977.jhb@freebsd.org>
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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@des.no> 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
made.
Alan
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