Re: [PATCH v1 2/3] mm/gup: use gup_can_follow_protnone() also in GUP-fast

[Jason Gunthorpe]

On Tue, Aug 30, 2022 at 08:53:06PM +0200, David Hildenbrand wrote:
> On 30.08.22 20:45, Jason Gunthorpe wrote:
> > On Tue, Aug 30, 2022 at 08:23:52PM +0200, David Hildenbrand wrote:
> >> ... and looking into the details of TLB flush and GUP-fast interaction
> >> nowadays, that case is no longer relevant. A TLB flush is no longer
> >> sufficient to stop concurrent GUP-fast ever since we introduced generic
> >> RCU GUP-fast.
> > 
> > Yes, we've had RCU GUP fast for a while, and it is more widely used
> > now, IIRC.
> > 
> > It has been a bit, but if I remember, GUP fast in RCU mode worked on a
> > few principles:
> > 
> >  - The PTE page must not be freed without RCU
> >  - The PTE page content must be convertable to a struct page using the
> >    usual rules (eg PTE Special)
> >  - That struct page refcount may go from 0->1 inside the RCU
> >  - In the case the refcount goes from 0->1 there must be sufficient
> >    barriers such that GUP fast observing the refcount of 1 will also
> >    observe the PTE entry has changed. ie before the refcount is
> >    dropped in the zap it has to clear the PTE entry, the refcount
> >    decr has to be a 'release' and the refcount incr in gup fast has be
> >    to be an 'acquire'.
> >  - The rest of the system must tolerate speculative refcount
> >    increments from GUP on any random page
> > > The basic idea being that if GUP fast obtains a valid reference on a
> > page *and* the PTE entry has not changed then everything is fine.
> > 
> > The tricks with TLB invalidation are just a "poor mans" RCU, and
> > arguably these days aren't really needed since I think we could make
> > everything use real RCU always without penalty if we really wanted.
> > 
> > Today we can create a unique 'struct pagetable_page' as Matthew has
> > been doing in other places that guarentees a rcu_head is always
> > available for every page used in a page table. Using that we could
> > drop the code in the TLB flusher that allocates memory for the
> > rcu_head and hopes for the best. (Or even is the common struct page
> > rcu_head already guarenteed to exist for pagetable pages now a days?)
> > 
> > IMHO that is the main reason we still have the non-RCU mode at all..
> 
> 
> Good, I managed to attract the attention of someone who understands that machinery :)
> 
> While validating whether GUP-fast and PageAnonExclusive code work correctly,
> I started looking at the whole RCU GUP-fast machinery. I do have a patch to
> improve PageAnonExclusive clearing (I think we're missing memory barriers to
> make it work as expected in any possible case), but I also stumbled eventually
> over a more generic issue that might need memory barriers.
> 
> Any thoughts whether I am missing something or this is actually missing
> memory barriers?

I don't like the use of smb_mb very much, I deliberately choose the
more modern language of release/acquire because it makes it a lot
clearer what barriers are doing..

So, if we dig into it, using what I said above, the atomic refcount is:

gup_pte_range()
  try_grab_folio()
   try_get_folio()
    folio_ref_try_add_rcu()
     folio_ref_add_unless()
       page_ref_add_unless()
        atomic_add_unless()

So that wants to be an acquire

The pairing release is in the page table code that does the put_page,
it wants to be an atomic_dec_return() as a release.

Now, we go and look at Documentation/atomic_t.txt to try to understand
what are the ordering semantics of the atomics we are using and become
dazed-confused like me:

 ORDERING  (go read memory-barriers.txt first)
 --------

  - RMW operations that have a return value are fully ordered;

  - RMW operations that are conditional are unordered on FAILURE,
    otherwise the above rules apply.

 Fully ordered primitives are ordered against everything prior and everything
 subsequent. Therefore a fully ordered primitive is like having an smp_mb()
 before and an smp_mb() after the primitive.

So, I take that to mean that both atomic_add_unless() and
atomic_dec_return() are "fully ordered" and "fully ordered" is a super
set of acquire/release.

Thus, we already have the necessary barriers integrated into the
atomic being used.

The smb_mb_after_atomic stuff is to be used with atomics that don't
return values, there are some examples in the doc

Jason