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.. Jason
