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

C. Jayachandran c.jayachandran at gmail.com
Wed Jun 2 12:17:02 UTC 2010


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

> 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.

Thanks,
JC.


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