mips pmap patch

[Jayachandran C.]

On Sat, Aug 11, 2012 at 11:18 PM, Alan Cox <alc at rice.edu> wrote:
>
> On 08/09/2012 10:36, Jayachandran C. wrote:
>
> On Wed, Aug 8, 2012 at 9:40 PM, Alan Cox <alc at rice.edu> wrote:
>>
>> Can someone please test this patch?  It applies some changes to the mips pmap that were made a long time ago to the amd64 and i386 pmaps.  In particular, it reduces the size of a pv entry.
>>
>> Briefly, the big picture is that in order to move forward with further locking refinements to the VM system's machine-independent layer, I need to eliminate all uses of the page queues lock from every pmap.  In order to remove the page queues lock from the mips pmap, I need to port the new pv entry allocator from the amd64 and i386 pmaps.  This patch is preparation for that.
>
>
> Tested the patch on XLP for about an hour ('make -j 64 buildworld' on 32 cpu mips64) and did not see any issues.
>
>
> Thank you for the quick response.  I am attaching the next patch for testing.
>
> This patch does two things:
>
> 1. It ports the new PV entry allocator from x86.  This new allocator has two virtues.  First, it doesn't use the page queues lock.  Second, it shrinks the size of a PV entry by almost half.
>
> 2. I observed and fixed a rather serious bug in pmap_remove_write().  After removing write access from the physical page's first mapping, pmap_remove_write() then used the wrong "next" pointer.  So, the page's second, third, etc. mapping would not be write protected.  Instead, some arbitrary mapping for a completely different page would be write protected, likely leading to spurious page faults later to reestablish write access to that mapping.
>
> This patch needs testing in both 32 bit and 64 bit kernels.

Ran the compile test on 32 and 64 bit kernels, and did not see  any issue.

I could not test for more than an hour on 32-bit due to another
problem (freelist 1 containing direct-mapped pages runs out of pages
after about an hour of compile test).  This issue has been there for a
long time, I am planning to look at it when I get a chance.

JC.