Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

[Nick Piggin <npiggin@suse.de>]

On Wed, Mar 07, 2007 at 10:32:22AM +0100, Peter Zijlstra wrote:
> On Wed, 2007-03-07 at 01:07 -0800, Andrew Morton wrote:
> > On Wed, 07 Mar 2007 09:51:57 +0100 Miklos Szeredi <miklos@szeredi.hu> wrote:
> > 
> > > > > Dirty page accounting doesn't work either on
> > > > > non-linear mappings
> > > > 
> > > > It doesn't?  Confused - these things don't have anything to do with each
> > > > other do they?
> > > 
> > > Look in page_mkclean().  Where does it handle non-linear mappings?
> > > 
> > 
> > OK, I'd forgotten about that.  It won't break dirty memory accounting,
> > but it'll potentially break dirty memory balancing.
> > 
> > If we have the wrong page (due to nonlinear), page_check_address() will
> > fail and we'll leave the pte dirty.  That puts us back to the pre-2.6.17
> > algorithms and I guess it'll break the msync guarantees.
> > 
> > Peter, I thought we went through the nonlinear problem ages ago and decided
> > it was OK?
> 
> Can recollect as much, I modelled it after page_referenced() and can't
> find any VM_NONLINEAR specific code in there either.
> 
> Will have a hard look, but if its broken, then page_referenced if
> equally broken it seems, which would make page reclaim funny in the
> light of nonlinear mappings.

page_referenced is just an heuristic, and it ignores nonlinear mappings
and the page which will get filtered down to try_to_unmap.

Page reclaim is already "funny" for nonlinear mappings, page_referenced
is the least of its worries ;) It works, though.