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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 11 Sep 2020 09:45:33 -0700
Message-ID: <CAHk-=wiiEUD_XvNnXysYsKiT4B3SajWbZ4VKY3jYk-17EEaaiA@mail.gmail.com>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding
To:     Alexander Gordeev <agordeev@linux.ibm.com>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, John Hubbard <jhubbard@nvidia.com>,
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On Fri, Sep 11, 2020 at 5:20 AM Alexander Gordeev
<agordeev@linux.ibm.com> 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