Re: [PATCH v2 6/9] mm: free user PTE page table pages

[Qi Zheng]

On 2021/9/1 PM11:32, Jason Gunthorpe wrote:
> > > Also, I don't really understand how this scheme works with
> > > get_user_pages_fast.
> >
> > With the RCU change it in #8 it should work just fine, because RCU
> > synchronize has to wait either until all other CPUs have left the RCU read
> > section, or re-enabled interrupts.
>
> So at this point in the series fast gup is broken, that does mean the
> series presentation really needs to be reworked. The better
> presentation is to add the API changes, with a
> no-functional-difference implementation, push the new API in well
> split patches to all the consumption sites, then change the API to
> have the new semantics.
>
> RCU and refcount to free the page levels seems like a reasonable
> approach, but I have to say I haven't thought it through fully - are
> all the contexts that have the pte deref safe to do call_rcu?

See Documentation/RCU/rcubarrier.rst:

"Since call_rcu() never blocks, this code can safely be used from within
IRQ context."

So I think call_rcu() can be safely run in any context.

Thinks,
Qi

> Jason