svn commit: r208589 - head/sys/mips/mips

[Alan Cox]

C. Jayachandran wrote:
> On Wed, Jun 2, 2010 at 3:23 AM, Alan Cox <alc at cs.rice.edu> wrote:
>   
>> On 5/27/2010 5:05 AM, Jayachandran C. wrote:
>>     
>>> Author: jchandra
>>> Date: Thu May 27 10:05:40 2010
>>> New Revision: 208589
>>> URL: http://svn.freebsd.org/changeset/base/208589
>>>
>>> Log:
>>>   Call VM_WAIT in pmap_ptpgzone_allocf() if  M_WAITOK is set.
>>>   Removed unused variable.
>>>
>>>   Approved by: rrs (mentor)
>>>
>>>
>>>       
>> I'm afraid that this will work some of the time, but not all of the time.
>>  Specifically, there is no guarantee that any of the available free (or
>> cached) pages after the VM_WAIT will fall within the range of suitable
>> physical addresses.  Moreover, and perhaps most worrisome, is that this
>> function could do a lot of spinning.  Every time this function sleeps and a
>> single page is freed (or cached) by someone else, this function will be
>> reawakened.  With a little bad luck, you could spin indefinitely.
>>
>> For essentially this reason, contigmalloc(), kmem_alloc_contig(), and
>> kmem_alloc_attr() don't use VM_WAIT, but instead a function called
>> vm_contig_grow_cache().
>>     
>
> I had seen the vm_contig_grow_cache() usage, but could not use it as
> it was defined as a static function.
>
> I did not want to use contigmalloc()/kmem_alloc_contig() either,
> because the pages in that memory region are already direct mapped and
> setting up another mapping is not necessary. The overall idea is to
> allocate pages in the direct mapped region for the page table entries
> so that we don't take a TLB exception while accessing page tables
> (which is costly as MIPS has a software TLB exception handler).
>
> Can you suggest the right way to do this? I will make the changes and
> send a patch for approval.
>   

I would encourage you to make vm_contig_grow_cache() an extern function 
and use it.  (vm/vm_pageout.h is probably the best place for the 
function prototype.)

If you're interested in taking this a small step further, it would make 
sense to add two parameters to vm_contig_grow_cache() and 
vm_contig_launder(): a "low" and a "high" physical address.  
vm_contig_launder() could then skip pages that are outside the desired 
range.

Regards,
Alan