Re: Improve page refcount scalability

[Kiryl Shutsemau <kirill@shutemov.name>]

On Tue, Oct 13, 2026 at 10:03:12PM +0100, Matthew Wilcox wrote:
> On Tue, Oct 13, 2026 at 11:43:14AM -0700, Linus Torvalds wrote:
> > The fairly short series of ten patches basically enhances folio lookup
> > by noticing when folios are hot, and setting a "delay relasing this
> > folio until after a RCU period", and then using that RCU delay it does
> > not just the folio lookup itself under RCU (which we've done forever),
> > but also the actual folio data accesses themselves under RCU. No
> > refcount taking needed for the fastpath where we can just read the
> > data from the folio entirely in an RCU-protected region when the user
> > copy doesn't cause page faults.
> > 
> > Unlike my original approach - because it now actually delays the
> > freeing of the underlying folio data - there's no need for any double
> > buffering, and a lot of the original limitations of my original patch
> > go away too. That improves performance quite a lot. And by "quite a
> > lot"  I mean "I see a 23x performance increase on my Ryzen
> > threadripper". Admittedly that is a very artificial case of reading
> > the same small part of a page over and over in a threaded loop on all
> > CPUs, so any real life impact will be much much less.
> > 
> > Anyway, I wasn't planning on posting the individual patches, I think
> > we all use git by now and probably find it easier than fetching
> > patches from the list. So it's all at
> > 
> >     git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux rcu-free-v3
> > 
> > and the commit messages are a bit more verbose than I would have done
> > it myself without tool use, but maybe it's better that way.
> 
> Obviously the numbers are compelling.  And thanks to Kiryl for figuring
> out a workload that confirmed my fears, then remedying the problem.
> 
> I don't like the PG_rcu_free flag.

I wonder if you would be okay with a 64-bit-only flag? Those are less
scarce. That's the direction I was looking at:

https://git.kernel.org/pub/scm/linux/kernel/git/kas/linux.git/commit/?id=59b23fdc4bb0d008e2a30d24156177eeede8a577

I don't think we care about the scalability of 32-bit machines here.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov