Re: [rfc] lockless get_user_pages for dio (and more)

["Siddha, Suresh B" <suresh.b.siddha@intel.com>]

On Mon, Oct 15, 2007 at 10:25:11PM +1000, Nick Piggin wrote:
> 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...

I am reasonably sure, it will perform better than mine, as it addresses
the mmap_sem cacheline bouncing also.

I think Brian/Badari can help us out in getting the numbers.

thanks,
suresh

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