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

[Alan Cox]

C. Jayachandran wrote:
> On Wed, Jun 2, 2010 at 11:57 AM, Alan Cox <alc at cs.rice.edu> wrote:
>   
>> 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.)
>>     
>
> I'll create a patch for this and related changes in mips/mips/pmap.c
>
>   

Great.  Thanks!

>> 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.
>>     
>
> I am looking at this now,  part of the logic which
> vm_phys_alloc_contig() uses to check pages address should work here.
> Will send out changes for this, if it works out.
>
>   

I suspect that you'll just need to add two or three lines to 
vm_contig_launder().  If something doesn't make sense, just e-mail me.

Regards,
Alan