On Fri, Sep 11, 2020 at 5:20 AM Alexander Gordeev
[off-list ref] wrote:

What if the entry is still pud_present, but got remapped after
READ_ONCE(*pudp)? IOW, it is still valid, but points elsewhere?

That can't happen.

The GUP walk doesn't hold any locks, but it *is* done with interrupts
disabled, and anybody who is modifying the page tables needs to do the
TLB flush, and/or RCU-free them.

The interrupt disable means that on architectures where the TLB flush
involves an IPI, it will be delayed until afterwards, but it also acts
as a big RCU read lock hammer.

So the page tables can get modified under us, but the old pages won't
be released and re-used.

                Linus