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

Andrew Morton
2007-03-07T09:26:54Z

On Wed, 7 Mar 2007 10:18:23 +0100 Nick Piggin <npiggin@suse.de> wrote:

> On Wed, Mar 07, 2007 at 01:07:56AM -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?
>
> msync breakage is bad, but otherwise I don't know that we care about
> dirty page writeout efficiency.

Well. We made so many changes to support the synchronous
dirty-the-page-when-we-dirty-the-pte thing that I'm rather doubtful that
the old-style approach still works. It might seem to, most of the time.
But if it _is_ subtly broken, boy it's going to take a long time for us to
find out.

> But I think we discovered that those msync changes are bogus anyway
> becuase there is a small race window where pte could be dirtied without
> page being set dirty?

Dunno, I don't recall that. We dirty the page before the pte...