On Wed, Feb 16, 2005 at 12:15:36PM -0500, Stephan Uphoff wrote:
> On Tue, 2005-02-15 at 21:49, Chuck Silvers wrote:
> > On Tue, Feb 15, 2005 at 05:00:09PM -0500, Stephan Uphoff wrote:
> > > Hi Chuck,
> > > 
> > > looks great !
> > > 
> > > You may want to call pmap_remove for failed pmap_enter calls
> > > (or remove PMAP_CANFAIL?) to prevent stale page table entries.
> > 
> > good point, it's not really specified whether a failed pmap_enter(PMAP_CANFAIL)
> > that was replacing an existing mapping is required to remove the old mapping
> > or not.  we appear to have different behaviour on different platforms.
> > 
> > I would say that it should always remove the old mapping, since otherwise
> > the caller will have to take care of it explicitly anyway.  if everyone
> > agrees on this, I'll clarify this in the manpage and see about fixing
> > the implementations.
> 
> I kind of like the current behaviour (On some platforms :-( ?).
> 
> It allows trying to replace one wired mapping with another.
> If pmap_enter fails the operation can be retried later with the old
> mapping still intact.
> 
> Frank van der Linden requested that mappings must be changed atomically
> (no time where no page is mapped in the VA) when I did some SMP fixes to
> the i386 pmap.
> I think he mentioned wired pages when he requested the change but this
> was a long time ago.

well, the reason why PMAP_CANFAIL would fail is that it needs to allocate
memory (such as for a page-table page or a pv entry) but there is no memory
available.  if there was previous mapping, then the new mapping can reuse
the resources that the old mapping was using, so it shouldn't fail.
I think all the existing implementations can easily accomodate this
(though some of them don't work this way currently), and it seems
reasonable to assert that any pmap implementation could work this way.

so I think we can require both that a failed pmap_enter(PMAP_CANFAIL)
should not leave an old mapping behind, and that entering a wired mapping
on top of an existing wired mapping should be atomic as you describe
and should never fail.

-Chuck