Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding

From: Linus Torvalds
Date: Fri Sep 11 2020 - 12:54:16 EST

On Fri, Sep 11, 2020 at 5:20 AM Alexander Gordeev
<agordeev@xxxxxxxxxxxxx> 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.