Freeing page tables through RCU

[Matthew Wilcox]

On Fri, Feb 26, 2021 at 10:42:00AM -0400, Jason Gunthorpe wrote:
> On Thu, Feb 25, 2021 at 08:58:20PM +0000, Matthew Wilcox wrote:
> 
> > I'd like to hear better ideas than this.
> 
> You didn't like my suggestion to put a sleepable lock around the
> freeing of page tables during flushing?
> 
> I still don't see how you convert the sleepable page walkers to use
> rcu??

I don't want to convert the sleepable ones to use RCU ... I want to
convert the non-sleeping ones to use RCU.  A page_table_free_lock might
work, but it might have its own problems later (eg a sleeping lock can't
be acquired under RCU or spinlock, and it can't be a spinlock because
it'd have to be held while we wait for IPIs).

I think it would solve my immediate problem, and I wonder if it might
solve some other problems ...