Messages in this thread | | | Date | Tue, 30 Aug 2022 15:45:42 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH v1 2/3] mm/gup: use gup_can_follow_protnone() also in GUP-fast |
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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
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