On Monday 15 October 2007 04:19, Siddha, Suresh B wrote:
> On Sun, Oct 14, 2007 at 11:01:02AM +1000, Nick Piggin wrote:
> > On Sunday 14 October 2007 09:27, Nick Piggin wrote:
> > > On Saturday 13 October 2007 06:34, Siddha, Suresh B wrote:
> > > > sounds like two birds in one shot, I think.
> > >
> > > OK, I'll flesh it out a bit more and see if I can actually get
> > > something working (and working with hugepages too).
> >
> > This is just a really quick hack, untested ATM, but one that
> > has at least a chance of working (on x86).
>
> When we fall back to slow mode, we should decrement the ref counts
> on the pages we got so far in the fast mode.

Here is something that is actually tested and works (not
tested with hugepages yet, though).

However it's not 100% secure at the moment. It's actually
not completely trivial; I think we need to use an extra bit
in the present pte in order to exclude "not normal" pages,
if we want fast_gup to work on small page mappings too. I
think this would be possible to do on most architectures, but
I haven't done it here obviously.

Still, it should be enough to test the design. I've added
fast_gup and fast_gup_slow to /proc/vmstat, which count the
number of times fast_gup was called, and the number of times
it dropped into the slowpath. It would be interesting to know
how it performs compared to your granular hugepage ptl...