Re: [PATCH v1 0/4] smaps / mm/gup: fix gup_can_follow_protnone fallout

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, 27 Jul 2023 at 14:28, David Hildenbrand <david@redhat.com> wrote:
>
> This is my proposal on how to handle the fallout of 474098edac26
> ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()") where I
> accidentially missed that follow_page() and smaps implicitly kept the
> FOLL_NUMA flag clear by *not* setting it if FOLL_FORCE is absent, to
> not trigger faults on PROT_NONE-mapped PTEs.

Ugh.

I hate how it uses FOLL_FORCE that is inherently scary.

Why do we have that "gup_can_follow_protnone()" logic AT ALL?

Couldn't we just get rid of that disgusting thing, and just say that
GUP (and follow_page()) always just ignores NUMA hinting, and always
just follows protnone?

We literally used to have this:

        if (!(gup_flags & FOLL_FORCE))
                gup_flags |= FOLL_NUMA;

ie we *always* set FOLL_NUMA for any sane situation. FOLL_FORCE should
be the rare crazy case.

The original reason for not setting FOLL_NUMA all the time is
documented in commit 0b9d705297b2 ("mm: numa: Support NUMA hinting
page faults from gup/gup_fast") from way back in 2012:

         * If FOLL_FORCE and FOLL_NUMA are both set, handle_mm_fault
         * would be called on PROT_NONE ranges. We must never invoke
         * handle_mm_fault on PROT_NONE ranges or the NUMA hinting
         * page faults would unprotect the PROT_NONE ranges if
         * _PAGE_NUMA and _PAGE_PROTNONE are sharing the same pte/pmd
         * bitflag. So to avoid that, don't set FOLL_NUMA if
         * FOLL_FORCE is set.

but I don't think the original reason for this is *true* any more.

Because then two years later in 2014, in commit c46a7c817e66 ("x86:
define _PAGE_NUMA by reusing software bits on the PMD and PTE levels")
Mel made the code able to distinguish between PROT_NONE and NUMA
pages, and he changed the comment above too.

But I get the very very strong feeling that instead of changing the
comment, he should have actually removed the comment and the code.

So I get the strong feeling that all these FOLL_NUMA games should just
go away. You removed the FOLL_NUMA bit, but you replaced it with using
FOLL_FORCE.

So rather than make this all even more opaque and make it even harder
to figure out why we have that code in the first place, I think it
should all just be removed.

The original reason for FOLL_NUMA simply does not exist any more. We
know exactly when a page is marked for NUMA faulting, and we should
simply *ignore* it for GUP and follow_page().

I think we should treat a NUMA-faulting page as just being present
(and not NUMA-fault it).

Am I missing something?

                  Linus