Merging 64 bit changes to -HEAD - part 2

[Alan Cox]

On 6/19/2010 5:18 AM, Juli Mallett wrote:
> On Fri, Jun 18, 2010 at 12:41, Neel Natu<neelnatu at gmail.com>  wrote:
>    
>> Hi JC,
>>
>> But what you really want here is to eliminate the intr_disable() and
>> intr_restore() and keep sched_pin() and  sched_unpin().
>>      
> Are you sure?  I'm not.  By disabling interrupts we only have to
> ensure that the fault path on any address we might access within those
> routines doesn't need to use the large memory map.  This isn't
> trivial, but I think we can acquire a reasonable confidence about it.
> If we merely pin, we have to ensure that nothing else that can run
> (including interrupts and threads that run via preemption) that would
> access the large memory map — given that this includes routines like
> pmap_zero_page, I think there's good reason for caution.  Disabling
> interrupts is more conservative, but I think rightly-so.  I may be
> mistaken.
>    

You're not mistaken.  See, for example, the i386 pmap_zero_page().  
Pinning by itself is insufficient because a pinned thread can be 
preempted, and the thread that then runs (on the same processor) could 
call pmap_zero_page().  So, pinning must be combined with a 
per-processor mutex.

I can imagine that blocking interrupts on mips is cheaper than the 
combination of pinning and a mutex.  However, do you want to have 
interrupts blocked for the time it takes to read 4KB from DRAM and write 
4KB to DRAM for pmap_copy_page()?  Ultimately, that's the question that 
you need to answer.

Alan