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

From: John Hubbard
Date: Thu Sep 10 2020 - 17:23:50 EST

On 9/10/20 11:13 AM, Jason Gunthorpe wrote:
On Thu, Sep 10, 2020 at 10:35:38AM -0700, Linus Torvalds wrote:
On Thu, Sep 10, 2020 at 2:40 AM Alexander Gordeev
<agordeev@xxxxxxxxxxxxx> wrote:

It is only gup_fast case that exposes the issue. It hits because
pointers to stack copies are passed to gup_pXd_range iterators, not
pointers to real page tables itself.

Can we possibly change fast-gup to not do the stack copies?

I'd actually rather do something like that, than the "addr_end" thing.

As you say, none of the other page table walking code does what the
GUP code does, and I don't think it's required.

As I understand it, the requirement is because fast-gup walks without
the page table spinlock, or mmap_sem held so it must READ_ONCE the

It then checks that it is a valid page table pointer, then calls

The arch implementation of pXX_offset() derefs again the passed pXX
pointer. So it defeats the READ_ONCE and the 2nd load could observe
something that is no longer a page table pointer and crash.

Just to be clear, though, that makes it sound a little wilder and
reckless than it really is, right?

Because actually, the page tables cannot be freed while gup_fast is
walking them, due to either IPI blocking during the walk, or the moral
equivalent (MMU_GATHER_RCU_TABLE_FREE) for non-IPI architectures. So the
pages tables can *change* underneath gup_fast, and for example pages can
be unmapped. But they remain valid page tables, it's just that their
contents are unstable. Even if pXd_none()==true.

Or am I way off here, and it really is possible (aside from the current
s390 situation) to observe something that "is no longer a page table"?

John Hubbard