Re: [PATCH v7 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[David Hildenbrand]

On 02.05.23 20:59, Peter Zijlstra wrote:
> On Tue, May 02, 2023 at 07:34:06PM +0200, David Hildenbrand wrote:
> > Now, if we read folio->mapping after checking if the page we pinned is still
> > mapped (PTE unchanged), at least the page we pinned cannot be reused in the
> > meantime. I suspect that we can still read "NULL" on the second read. But
> > whatever we dereference from the first read should still be valid, even if
> > the second read would have returned NULL ("rcu freeing").
>
> Right, but given it's the compiler adding loads we're not sure what if
> anything it uses and it gets very hard to reason about the code.
>
> This is where READ_ONCE() helps, we instruct the compiler to only do a
> single load and we can still reason about the code.

I completely agree, and I think we should fix that in 
page_is_secretmem() as well.

--
Thanks,

David / dhildenb