init_pte_prot() patch

[Juli Mallett]

Hi Alan,

On Wed, Jun 2, 2010 at 21:59, Alan Cox <alc at imimic.com> wrote:
> I would appreciate it if someone would test the attached patch.  (A
> "buildworld" would probably suffice.)  Essentially, it does two things:

Sorry I didn't respond to this when you sent it to me.

> 1. The virtual memory system only cares about the contents of a page's dirty
> field if that page is managed (i.e., it is pageable).  And, in fact, if you
> survey the calls to vm_page_dirty() in the MIPS or any other pmap, they are
> generally conditioned on the mapping having been for a managed page.
>
> The MIPS pmap_enter() is an exception to this rule.  It is unconditionally
> calling vm_page_dirty() on any page mapped within the kernel address space,
> managed or otherwise.  In fact, it is highly unusual for pmap_enter() to be
> calling vm_page_dirty() on the page being mapped, regardless of whether it
> is managed.  This call to vm_page_dirty() shouldn't be needed if change #2
> below is also made.  The attached patch eliminates the call.

I believe that the reason the MIPS pmap does that is because
PTE_RWPAGE includes PTE_M which is the TLB dirty bit.  This means that
we won't get exceptions when that page is modified, and so MIPS is
pre-dirtying the VM page to allow it to make that optimization.  At
least, that's what the intent appears to be.  Whether that's a correct
model for the interaction between pmap and the VM system's management
of those kernel pages, I don't know.

Juli.